iPhone 5 searching for carrier signal all the time..!

Ocassionally there is some signal but only for a very short period of time: after 10-15 minutes it goes back to searching... I have tried everything already - I reseted the network settings, all settings, made a hard reset, installed updates, tried different SIM-cards..! It does not matter what I do, nothing changes. I already googled for two days straight and I have not found a clue whatsoever! Is it possible that somehow the antenna inside the phone is broken?? If yes, is there something I could do to fix it? Thank you very much for your answers!!