Hi @ccskymade
Welcome to the IPFire community.
See the following posts on this topic
https://community.ipfire.org/t/4g-lte-failover/4372/7
https://community.ipfire.org/t/wan-backup-line/5863/4
Unfortunately the backup has to be dialup and cannot be dhcp
I believe this is planned to be capable in IPFire 3.x but that is still some way away.