I think the obvious thing is to disassemble the device and put back together methodically ensuring that all connectors and antennas are replaced properly and that the antennas are connect to the correct points on the board and not swapped over.
Factory reset the device, update the software and test again.
If none of that works I would assume that there is some damage to the bottom board and would swap that out. Once swapped out I would software the device again and then test.
If all of that fails it really needs to go to a professional who may diagnose a PCB fault.