Signal drops and mobile data not working

Hello,

I had an iPhone 6 in a pretty bad cosmetically condition, and have decided to swap the housing.

After the reassembly everything is still working, except the internet connection when sing mobile data. And the problem only appears when inside a building.

iPhone shows 3G but nothing loads, however it does when connected to wifi. The signal also seems to be weak and I sometimes even lose it for a short amount of time.

I have already reset the phone and the network settings, deleted all the profiles and tested the SIM card. The SIM card seems to be working fine and none of the above steps helped.

Does anyone know what could the problem be and how to fix it?

Best regards

Urban