I solved this problem on my S8 Plus. After changing the screen the network was no longer working, then I tried to press the lower part of the phone (near the home button) and the 4g network started working, I disassembled the phone, on the second board(the smaller one) there are some pins, which touch the plastic shell, with tweezers I lifted them ( a little bit ?) to make better electrical contact, I cleaned them with acetone and a piece of paper. then I reassembled the phone and it is now working perfectly! These pins touch some metal pieces in the plastic shell, to boost the antenna signal i tink.