4G backup internet: how failover works and how much data you need
When the broadband drops, a router with a 4G backup can keep card payments, phones and cloud apps going until it returns. This guide covers how the switch works, what to plan for, and how to test it before a real outage.
How router failover works
A failover router has two ways out to the internet, usually broadband and a 4G SIM. You rank them, and the router uses the highest-priority connection that is working [1].
It decides with a health check. On the Teltonika RUT241 the check pings a list of hosts (by default 1.1.1.1 and 8.8.8.8) every 3 seconds. Three failed tests in a row mark a connection offline, and three good tests mark it online again [1]. Two details are easy to miss: the check is off by default, and with no hosts listed a connection is always treated as up [1]. Without it, a line that is connected but carrying nothing may never trigger the switch.
A typical outage then runs like this:
- Normal running. Broadband carries the traffic; 4G waits.
- Broadband fails. With the defaults above, three failed tests 3 seconds apart take about 9 seconds (our arithmetic) before the router marks the line offline and moves traffic to 4G [1].
- Open connections. Connections that were already open may stay on the old path. Teltonika’s “flush connections” option makes existing traffic re-check which connection to use after an event you choose [1].
- Broadband returns. After three good tests the line counts as online, and the router goes back to it because it always prefers the highest-priority working connection [1].
The same router offers load balancing instead, which spreads connections across both links but does not speed up any single one [1]. With a capped SIM, failover suits better, since load balancing puts everyday traffic on 4G.
What keeps working on backup, and what to watch
Card terminals
Adyen’s terminal documentation, as one example, says payments can continue over cellular if the main connection fails, and recommends automatic cellular failover on the main router or terminals with built-in cellular. It also says firewalls should allow its domains by name rather than IP address, because addresses change [8]. Offline payments are only available in some set-ups, and the merchant is liable for failed captures and chargebacks on them [8], so a working backup is the safer plan. Ask your own payment provider for its requirements.
Phone calls over the internet
Cisco’s figures for two common voice codecs: G.711 sends 50 packets a second with 160 bytes of voice each, and G.729 sends 50 a second with 20 bytes each, both plus a 40-byte IP/UDP/RTP header [9]. Over a mobile link that works out (our arithmetic) at about 80 kbit/s each way for G.711, or 72 MB for an hour-long call counting both directions. G.729 comes to about 24 kbit/s each way, or 21.6 MB an hour.
A call in progress may drop when the router switches, because traffic then leaves from a different address. Microsoft’s Teams guidance asks for firewalls not to change NAT mappings for UDP during a session [10], and a switch of connection does exactly that. Ask your phone provider how handsets re-register after a switch, and whether it only accepts traffic from your broadband’s fixed IP address.
CCTV remote viewing
Live viewing uses data quickly. Reolink’s figures for several of its 4G cameras give a default main-stream bitrate of 2 Mbps, about 1 hour of viewing per GB, and a 672 kbps sub-stream, about 3.5 hours per GB [12]. Recording to the camera’s own SD card uses no data [12]. On backup, view the sub-stream and pause cloud uploads where the recorder allows it.
Remote access can also fail. Some mobile networks put SIMs behind carrier-grade NAT. The IETF reserved 100.64.0.0/10 for the link between that equipment and the customer’s [13], so a mobile address in that range means port forwarding is unlikely to reach the site. Use the recorder maker’s cloud service or a VPN instead; our CCTV router guide covers the set-up.
Cloud apps and laptops
Microsoft’s recommended per-user figures for Teams, in kilobits per second up/down, are 58/58 for audio, 1,500/1,500 for one-to-one video and 2,500/4,000 for meetings. Actual use varies, and Teams protects audio over video when bandwidth is short [10].
Laptops do not know the office is on 4G. Windows treats cellular connections as metered by default, but Wi-Fi and Ethernet networks must be set as metered by hand; on a metered connection Windows Update downloads only priority updates [11]. If backup data is tight, set key PCs to metered.
How much backup data do you need?
Work it out per outage day, then per month:
- List what must keep working, and what can wait.
- Estimate the data for each, from the figures above or the supplier’s.
- Multiply by the outage time you want to cover in a month.
- Add the router’s own background traffic, then leave headroom.
Background traffic includes the health check, if you switch it on for the 4G connection as well. With the defaults on the RUT241 page (two hosts, one ping each, every 3 seconds) [1], each ping and each reply carries at least a 20-byte IPv4 header [14] and an 8-byte ICMP echo header [15]. That alone is about 3.2 MB a day, or 97 MB in 30 days, before any ping payload (our arithmetic). A 30-second interval on the 4G connection cuts it to about 10 MB.
Worked examples (sourced figures, our assumptions about use):
| Example site | Assumed use per outage day | Our estimate |
|---|---|---|
| Shop with one internet phone line and card terminals | One hour of G.711 calls in total, plus the terminals | About 72 MB, plus what your payment provider says the terminals use |
| Office with five staff on Teams | Two hours of audio calls each, plus one hour of one-to-one video | About 0.5 GB of audio and 1.35 GB of video: roughly 1.9 GB |
| Site where the owner checks four cameras | Sub-streams for 15 minutes, twice a day, over a two-day outage | About 1.2 GB in total, or about 3.6 GB on the 2 Mbps main streams |
Phones and payments are light; video is heavy. For the shop, covering one outage day a month with a 30-second health check, the 500MB plan leaves room for the terminals. Longer or more frequent outages point to the 1GB plan. An office on video calls, or anyone viewing cameras, needs the 5GB plan, and two office days like the one above would use most of it.
Teltonika routers can also cap the SIM: a data limit switches mobile data off when reached, resetting daily, weekly or monthly [3]. On a backup link a hard stop may be worse than running over, so choose carefully. The same screen can send an SMS warning [3], but SimFalcon SIMs do not send or receive SMS [19], so use another alert method.
Choosing the router
For broadband plus 4G backup, a single-SIM router is often enough. The Teltonika RUT200 has one SIM slot and two Ethernet ports [5]. The RUT241 eSIM adds an eSIM; Teltonika says eSIM brings SIM switching to single-SIM-slot devices, with an option to switch back to the primary SIM after a timeout [4]. The RUT956 has two SIM slots, four Ethernet ports, RS232 and RS485 serial ports, and a digital input and output [6].
Dual SIM is not two live connections. Teltonika says most of its dual-SIM devices run one SIM at a time, the second acting as failover [2]. Switching follows rules you set, such as weak signal (default -90 dBm), no network or a failed data connection, checked every 30 seconds by default [3]. A second SIM makes most sense where 4G is the only connection, or the site cannot be offline at all.
See the RUT200, RUT241 eSIM and RUT956, or the backup internet range.
Antenna placement
Teltonika advises putting the antenna where signal is strongest, allowing for buildings, trees and walls, keeping connectors tight, and judging strength and quality together [7]. Its 4G bands:
| Rating | RSRP (strength) | SINR (quality) |
|---|---|---|
| Excellent | -80 dBm or stronger | 20 dB or more |
| Good | -80 to -90 dBm | 13 to 20 dB |
| Fair to poor | -90 to -100 dBm | 0 to 13 dB |
| Poor | weaker than -100 dBm | 0 dB or less |
ComReg notes that some building materials, especially certain energy-efficient insulation and windows, reduce the signal getting in. It suggests its SiteViewer map for finding nearby masts when positioning an external antenna [16]. If you are thinking of a repeater, ComReg says only devices that meet its standards and carry the CE mark are permitted; a repeater needs outdoor signal to work, and non-compliant devices can be seized, with fines of up to €5,000 on summary conviction [16]. Our weak or no network guide goes further.
Testing the failover
- Agree a quiet time with the site.
- Check the 4G connection on its own first: signal readings, and that it passes traffic.
- Fail the broadband the realistic way. Unplugging the router’s WAN cable only tests a dead cable. Disconnecting the line into the broadband modem leaves the modem powered but with no internet, which is what the health check is for [1].
- Time the switch. With the default settings it should take seconds, not minutes [1].
- Take a test payment, make a call, open a cloud app and view a camera.
- Reconnect the broadband; confirm the router moves back and phones re-register.
- Note the data used from the router’s counter, and repeat the test after any change to the router, broadband or phones.
Common mistakes
- Health check off, or no hosts listed. The connection is then treated as up whatever happens [1].
- Reliability set higher than the number of hosts. Teltonika warns the connection will then always show offline [1].
- PCs not set to metered. Updates and syncs run over the backup [11].
- No power backup. Without a UPS on the router, modem and switch, a power cut is an outage 4G cannot cover.
- Router behind walls or in a cabinet. Obstructions weaken the signal [7]; move it or fit an external antenna.
Where SimFalcon fits
SimFalcon supplies managed multi-network IoT SIMs, with data plans from 10MB to 10GB a month. Each plan is 12 months of service paid upfront, then renews yearly by Direct Debit [19]. Irish data plans can be delivered to other euro-area countries [19]. See the backup internet range, or find your SIM.
Sources
- [1] Teltonika, RUT241 failover, checked 22 October 2026.
- [2] Teltonika, Dual-SIM support, checked 22 October 2026.
- [3] Teltonika, RUT956 mobile settings, checked 22 October 2026.
- [4] Teltonika, RUT241 eSIM, checked 22 October 2026.
- [5] Teltonika, RUT200 interfaces, checked 22 October 2026.
- [6] Teltonika, RUT956 interfaces, checked 22 October 2026.
- [7] Teltonika, Mobile signal strength recommendations, checked 22 October 2026.
- [8] Adyen, Point-of-sale network configuration, checked 22 October 2026.
- [9] Cisco, Bandwidth consumption for voice calls, checked 22 October 2026.
- [10] Microsoft, Prepare your network for Teams, checked 22 October 2026.
- [11] Microsoft, Metered connections in Windows, checked 22 October 2026.
- [12] Reolink, Data use of Reolink 4G cameras, checked 22 October 2026.
- [13] IETF, RFC 6598, checked 22 October 2026.
- [14] IETF, RFC 791, checked 22 October 2026.
- [15] IETF, RFC 792, checked 22 October 2026.
- [16] ComReg, Mobile phone repeaters, checked 22 October 2026.
- [19] SimFalcon, IoT data SIM 500MB (Ireland), checked 22 October 2026.
Frequently asked questions
How quickly does a 4G backup take over?
It depends on the router's health check. With the defaults on Teltonika's RUT241 failover page, a test every 3 seconds and three failures to go offline, the router marks the broadband offline after about 9 seconds. A call in progress may drop when it switches.
Will card machines work on a 4G backup?
They can. Adyen, for example, recommends automatic cellular failover on the main router as one way to keep taking payments when the main connection fails. Check your own payment provider's network requirements, and take a test payment when you test the failover.
How much data does a 4G backup need?
Work it out from what must keep running. On our arithmetic from Cisco's figures, an hour of G.711 calls uses about 72 MB counting both directions, while Reolink says 1 GB gives about one hour of 2 Mbps camera viewing. Phones and payments are light; video is heavy.
Do I need a dual-SIM router for backup internet?
Not always. With broadband as the main line, one SIM is often enough. Teltonika says most of its dual-SIM routers use one SIM at a time with the second as failover, so dual SIM suits sites where 4G is the only connection or downtime is not acceptable.
Why can't I view my cameras remotely on the backup?
If the mobile network puts the SIM behind carrier-grade NAT, port forwarding will not reach the site from outside. Use the camera or recorder maker's cloud service, or a VPN, instead.
Questions about connectivity for your equipment? Talk to SimFalcon or ask Engineer Support.