Chosen Solution
I got a used iPhone SE with “broken cellular data but everything else fine”. It does seem fine, just some faded spots on the screen and cosmetic scuffs, and I thought the cell service fix would be easy. I swapped the cellular antenna (and loudspeaker) with the corresponding part from my functioning iPhone 5s, but this did not solve the issue. (In fact, the SE cellular antenna works great in my 5s, so I know it was not the antenna’s fault.) I do not think this is the carrier’s fault. I called T-mobile and they said the SIM was activated and should be working fine. I am able to make and receive calls through WiFi calling using the SIM card too. Any ideas as to what could be going wrong, or how I could diagnose? I have a working iPhone 5s, so I can swap parts in and out (if they’re compatible) as needed to check if they are working.
Try dialing *#06# on the numeric pad, you should be greeted with the IMEI number. If that doesn’t happen it is a Baseband chip issue. Trubleshooting a no service failure is among the most complicated jobs..half the phone is covered with chips handling communications and they are all interconnected one way or the other.
