That might indicate a problem with internal contacts between modules. If so, the best thing to do is to get out that little screwdriver that came with the phone, and disassemble the phone following the iFixit teardown and then clean all the contacts of the modules (but especially bottom module and motherboard and also contacts of the battery emplacement and of the battery itself) using isopropyl or very high-grade medical use alcohol (90%).
[Edit: use a cotton bud and leave plenty of time to dry, even if it’s alcohol: 3h]
When disassembling, no need to go beyond step 7 (i.e. no need to disassemble modules themselves).
The Vienna angels already signalled above may be able to assist, if you’re not very confident - or go along to that meetup mentioned by Urs just above 