[isostatic]

If I wanted a seemless non-SPOF network for my family, I'd put in two mikrotiks, with the primary on mains, and secondary on UPS, £120 for a pair to do routing at a decent (1gig) speed on the main, and built in 4G on the reserve.

Then I'd put the primary router on the wired line, the other one on a 4G sim which did nothing but heartbeats unless the wired line went down. If the wired line shut down, traffic would reroute via 4G within 10 seconds or so. If the primary router went down, the backup router would take over in a similar time frame. Might put some capping on the 4G router to the netflix/etc boxes to keep bandwidth costs down.

UPS would be about 10W, so £45 for a 4 hour one. Possibly look at renewable energy of some sort to keep the UPS going during an extended outage.

I'd then VRRP on the lan side with primary on the main router (which would have a backup route via the secondary router)

Cloud based VM to do monitoring/alerting and land outgoing openvpn tunnels from both routers to allow secure remote access.

£170, £10 a month plus main ISP, and an hour of config.

However in reality having an ISP provided router and showing them how to tether in a problem works fine. OK, they lose their devices if the main circuit goes off, but running those over 4G can be pricey.