iPhone 4 "No Service"/"Searching..." Problem!

I recently replaced my cracked iPhone 4 front and back screen with a cool orange color conversion! When I sent it to a technology store, I had put it on airplane mode for privacy reasoning. When I got it fixed and opened it back in the mail, I noticed there was "No Service" or "Searching...". But later the service came back 10 minutes from my home! But halfway home I lost it 😟. I got confused and fustrated so I looked up possible reasoning and said there could be problems with the SIM card, antenna, and pressure contact. The store said that it was the same when they had gotten it. I've tried so many possible ways (Resetting the whole device, turning device on/off, toggling the Airplane mode on/off, etc) and NONE HAS WORKED! Any possible reasons on fixing it?!? Thanks 🙂!