SIM card detected, but no service
I have gotten my hands on an old, out-of-warranty OnePlus One which can read/detect a SIM card, but it can't get service on it. I tried changing it from an older SIM to a newer, higher capacity one, but that didn't do anything, so the problem is with the device itself. I tried to get it fixed from some local repair shops, but they said that there's a problem with some particular SIM-related component and that's difficult to come by.
I'm interested in trying to fix this myself and learning about these things in the process. How can I troubleshoot this? How do I identify which part exactly is the problem? Where do I start?