Short answerPut a router with automatic failover in front of your fixed broadband, give it a business data SIM on the network that is strongest at your premises, and test the switch-over regularly. When the line drops, traffic moves to 4G in seconds, and moves back when the line returns.
Fixed broadband fails more often than people expect: a digger through a duct, a fault at the cabinet, a router that dies on a Friday night. For a shop with card terminals or an office on cloud phones, a few hours offline costs real money. 4G backup is the cheapest insurance against it.
How failover works
The router watches the main connection, usually by pinging a reliable address on the internet. If the pings fail for a set time, it switches traffic to the mobile connection. When the main line is healthy again, it switches back. Good routers do this in seconds and can keep the mobile link warm so the switch is quicker.
Two common layouts:
- One router does both: a business router with a WAN port for the broadband and a 4G modem for backup. Simple and tidy.
- A 4G router alongside your existing one: your main router uses the 4G router as a second WAN. Useful when you want to keep the ISP’s router.
Router Store has an overview of 4G and 5G office internet, and a range of office 4G routers.
Pick the network carefully
Choose the mobile network on signal at your premises, not on brand. Our signal guide shows how to read it. Two more points:
- A dual-SIM router with SIMs on two networks protects you against a mobile outage as well as a broadband one.
- If your broadband and the mobile mast share the same local infrastructure, a single incident can take both out. That is rare, but on critical sites it is worth asking.
Size the backup plan
Backup SIMs sit idle most of the month, so a modest allowance is usually enough. Make sure the plan does not stop dead at the limit, and set an alert in the router so you know when failover has happened. Some routers can send an email or SMS when they switch.
Things that catch people out
- Phones and VPNs: your public IP changes when you fail over. Cloud phone systems and site-to-site VPNs need to cope with that, or you need a fixed IP SIM on the backup.
- Card terminals: check they use the router rather than their own SIM, and that their traffic is allowed out on the backup link.
- Nobody tests it: unplug the broadband once a quarter and check everything still works.
Frequently asked questions
How quickly does 4G failover happen?
Typically seconds to a minute, depending on how the router detects the failure. Faster detection risks switching on a brief blip, so most people settle on 10 to 30 seconds.
Can 4G replace my broadband altogether?
In many places, yes. With a strong signal, 4G or 5G can be a sensible main connection, especially for rural sites and temporary premises.
Will my landline phones work on 4G backup?
Phones running over the internet (VoIP) will, if the router and phone system handle the address change. Traditional copper phone lines are being withdrawn; Router Store explains the PSTN switch-off.
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.