4G signal stopping after a week or so

Intermittent problems like this are hard to diagnose. By the sounds of it you have ruled out a parts and software issue. From experience if this started happening after an impact and it being intermittent I would be looking for:

·         A component on one of the RF lines has become loose from the pad.

·         A trace from the baseband IC has been severed.

·         A pad under the baseband CPU has become oxidised or cracked.

·         One of the pads under the sim holder has become oxidised or cracked.

In all these instances the temperature of the phone or the environment can cause it to go into no service mode. There are about 13 pages of schematics just on the RF lines and components which is why it will be extremely hard to find the fault as when you have the logic board out of its housing you are not sure if the problem is presenting itself at the time of measuring lines and components.