As @Tudor said, get yourself a router that supports failover.
You can either get one that has a second WAN input for a 4g/5g dongle, or one with a built in cellular facilitity. Draytek make both, but I'm sure there are others.
I prefered the separate 4g device because it allowed me to site it in the loft where it gets a better signal than at the router's location.
A couple of days ago I found the Virgin phoneline wasn't working. A glance at the BQM showed that the broadband had been down for a few hours. I had been working on line and with a Zoom call and hadn't noticed.