Short answerPick the network that is strongest where the router will sit, then a data-only business plan on that network with an allowance that fits your use. Add a fixed IP only if you need to connect in to the device, and a multi-network SIM if it moves around or cannot afford to drop out.
A data SIM is the part of a 4G installation people think about last, and it causes more problems than the router. The same router can run beautifully on one network and crawl on another a few miles away. This guide covers the decisions in the order that matters.
1. Choose the network before the plan
UK coverage maps are a starting point, not a promise. They show outdoor coverage, and a router inside a steel cabinet or a basement plant room sees something quite different. The most reliable check is a test on site: put a phone or a router with each network’s SIM where the router will live and read the signal values.
There are four networks with their own masts: EE, O2, Vodafone and Three. Vodafone and Three merged in 2025 and are combining their networks, but they still sell separate SIMs today. Every other brand you see, from giffgaff to the business resellers, runs on one of those four. Our guide to UK 4G networks explains who runs on what.
To read the signal properly, use RSRP, RSRQ and SINR rather than the bars on a screen. Our signal strength guide has a checker you can type the numbers into.
2. Data-only, and allowed in a router
Phone plans are priced for one person on one handset. Many forbid use in a router or tether, or apply a fair use limit that a busy office or a CCTV system will hit in days. For a router, use a data-only SIM sold for routers, mobile broadband or business data.
Things to check before you buy:
- Allowance and fair use. “Unlimited” plans usually have a fair use policy. Read it and check that router use is allowed.
- Contract length. 30-day rolling plans cost a little more but let you change network if the site turns out to be weak.
- What happens at the limit. Some plans stop, some slow down, some charge per extra gigabyte. A remote site that silently stops is worse than one that slows.
- Pooling. With several SIMs, a shared allowance stops one busy site running out while another sits idle.
More detail in 4G SIM-only data plans.
3. Do you need to connect in?
Most SIMs give the router a private address behind the network’s carrier-grade NAT. That is fine for browsing, email and anything that connects out. It stops you connecting in to a camera, recorder, PLC or the router itself.
If you need inbound access you have three choices: a fixed public IP SIM, a private APN that links the SIMs straight into your own network, or a VPN or cloud service such as Teltonika RMS that tunnels out from the router. CGNAT and remote access compares them.
4. Single network, multi-network or roaming
A standard SIM uses one network. A multi-network SIM can use more than one UK network and switches when the current one is lost. That is worth paying for on vehicles, temporary sites and anything where an outage costs more than the SIM.
Multi-network SIMs are usually IoT SIMs with a roaming profile. They tend to suit low and medium data use better than heavy streaming. See IoT and multi-network SIMs.
5. Match the SIM to the router
Check the SIM size (most routers take a mini SIM, and many SIMs come as a triple cut card), whether the router has one slot or two, and whether it supports eSIM. A dual-SIM router can hold SIMs from two different networks and fail over between them, which is often cheaper than a multi-network tariff.
Make sure the router supports the 4G bands the UK networks use. Our UK 4G bands guide lists them, and the router checklist covers the rest.
Common setups
| Use | Usual SIM | Read next |
|---|---|---|
| Office or shop internet | Business data SIM, large allowance | 4G backup broadband |
| CCTV and remote access | Fixed IP SIM or private APN | SIMs for CCTV |
| Vehicles and temporary sites | Multi-network SIM | IoT and multi-network SIMs |
| Meters and sensors | Pooled IoT SIM | LTE categories |
Frequently asked questions
Can I put a phone SIM in a 4G router?
It will usually connect, but many phone plans do not allow router or tethering use and may slow or block the connection. Use a data-only SIM that is sold for routers.
Which network is best for a 4G router?
The one that is strongest where the router will be. That changes from street to street, so test on site rather than relying on a national ranking.
Do I need a fixed IP SIM?
Only if you need to connect in to the router or something behind it, such as CCTV. For internet access alone, a standard data SIM is fine.
Will a 4G SIM work in a 5G router?
Yes. A 5G router uses 4G wherever 5G is not available. Some plans need 5G enabled on the account before the router will use 5G.
If you would rather someone matched the SIM to the site, send us the details and we will suggest a network and plan.
NANick Appleby
Nick Appleby has worked in UK telecoms since 1998, starting at BT, and has spent most of that time on cellular routers, data SIMs and the antennas that connect them for business and industrial sites. He writes 4G SIM to explain how the networks and the kit actually behave, in plain English.