I use three WAN connections at home, one fibre and two 4G. I don't use them specifically for failover as the primary reason though. Each of them is not the best speed wise so I set groups of devices to use each of them so everyone gets a snappy internet experience. If one fails then the groups defined will use one of the other connections though. This is all done with a pfSense firewall and once configured just does its thing automatically.
I have to say if I were paying a monthly subscription, as I do, just for a backup I think I'd not be happy. Wired broadband doesn't fail so often that a permanent solution that I have setup would be a worthwhile investment I think. I'd probably go down the phone tethering route for the odd occasion.