Cell service only works briefly after turning phone on.

Hello,

Not sure if anyone is paying attention to iPhone 5's anymore, but I have an interesting issue I would like to resolve. I have been trying to repair an iPhone 5, and have gotten it almost fully working. The issue is, the phone shows a cellular connection (ATT), but calls and data only work for a short period of time, then I get the call failed error even though the phone still shows a good connection. I have tried changing the cellular antenna, as well as taking the entire lightning port/speaker setup on the bottom from a working phone and subbing it in, to no avail. If I turn cell data/LTE off and on, I can sometimes get it to work, but turning the phone off and on always gives me about 5 minutes of connection.

Any ideas?

Thanks!