You should really check out an MVNO if you can afford to pay off your phone. You’ll save a LOT. I personally use Helium Mobile (uses Tmo and consumer decentralized network) but there are MVNOs that use AT&T if you prefer their coverage.
The major carriers overcharge for service since they lock people in with 0% financing.
Why would they not allow an mvno? Odd.
Mine is $6/mo unlimited since I was on the beta for Helium. It’s $20/mo now.