I just had a similar issue with my iPhone 6, searching and no service most of the time and if I get a signal eventually it only shows 3G (it should be 4G) and there is no data coming trough (works fine on WiFi). What I did is I replaced QFE1100 (the signal power ic) which was corroded due to water damage and now my phone works fine, I have 4G and I have SIM data. Could be the Baseband IC as well, you should reball or at least reflow it. Hope this helps someone :)