I have AT&T Fiber and 99% of the time it's fantastic, but there are several instances of 30-60 second downtime a day and I have a 5G modem with a Google Fi data sim as a backup. Failover is nearly-instant with a Unifi UDM.

The data sim costs nothing extra on top of my cellular plan and just counts towards my (already very generous) monthly limit of 50GB.