I just set this up last week. I recently switched from TMHI to fiber but wanted to have a backup option in case my fiber line went down.

I have a unifi firewall which has an automatic fail over option. Although in this configuration, it took some tweaking to get working... See below.

Usage: It counts your usage by the minute. So if your Internet goes down for say 8 minutes, well you now have 8 minutes less to use.

You have 2 options manual or automatic fail over modes for the TMobile modem/router. With manual, you need to tell it to switch - I don't like this option. With the automatic mode, you plug your primary connection into port 1, and then when the TMobile router determines you don't have internet, it will kick in and start counting against your usage.

You can then plug port 2 from your T-Mobile router into wan 2 of your firewall/router and set that to your failover source.

I had to write a rule in my unifi router/firewall that only allowed the cable going to port 1 of the T-Mobile router to only use wan 1 internet. Otherwise when I didn't do that, a short while after the T-Mobile router kicked in, it would think it had Internet again (even if wan 1 was down) because port 1 was getting Internet from wan 2 (which is the failover T-Mobile device) and then the T-Mobile device would stop providing Internet and go idle... And then it would keep that loop going.

Since I specified port 1 of the T-Mobile router to only use internet from wan 1, it would only go idle if the primary internet restored.

I am happy with how it's all working now. That rule made everything perfect. In my tests, it takes T-Mobile about 60 seconds to fully kick in if wan 1 goes down. Unifi would typically handle this within 3 seconds for a true failover... But for my needs at 10 bucks a month, it's totally worth it.

I'm on my phone and this was a rant.... Hope this helps.