I set up a backup cell connection to my cable internet connection. Sketchy Chinese 4G LTE modem. My router was a DIY job I set up off of Ubuntu Server. Everything ran to a Cisco switch and then was VLAN isolated. For the two WAN connections, I ran scripts from the router that periodically tried to reach out to several DNS providers and then average response rates to determine if the main connection was up. If not then it would modify default routes and push everything to the cell.
The cell connection had pretty low data cap, so it was just for backup and wasn’t a home style plan. I used the old TTL modification trick to get it to pass data like a phone. When I moved the backup to 5G, TTL modification stopped working and I had to resort to creating tunnel interfaces to an actual phone. Since that tunnel is limited in bandwidth to the lowest value, my speeds were really cut in half.