I think LTE has moved on a bit - you do get uncapped at very reasonable prices, and it would be a good backup solution regardless of your FNO.
Anyways, good luck!
You're probably better off doing this yourself by using a LTE backup line for failover. This is the only way to survive the family bleating about the Internet being down, and covers the case where the fiber line at your house is broken.
Could this be an issue on CI side instead of FF - perhaps the pppoe server is having an issue. I am certainly experiencing ppp disconnects (for whatever reason).
Whatever the technical people did made zero difference on my line - I am still having the same disconnect profile that was happening since 31 Dec @ 23:37.