FWIW, I have AT&T fiber, and got similar reasons, got T-Mobile 5G Home Backup Internet as a fallback about a month ago.
Documented that odyssey here:
It has worked well, and for $20 a month was a no-brainer. For anyone interested in doing this, the 2.4G radio in the TMo gateway can be turned off. They recommend leaving the 5g radio running.
