What carrier is the customer using? AT&T and the MNVOs who don’t use others as fallback are known to have coverage issues because of how bad their cellular coverage is known to be in some areas, especially with certain phones where the problem is made worse. Before you rule out the phone as a dud, try to use it on a carrier with good coverage in your area for 1 month and then the matching carrier for 1 month. If it works out, then the phone is fine and the customer has coverage issues. You’re better off waiting until you know it’s good before selling it again since it’s potentially defective.
If it isn’t the network, try and use the Apple repair program and get the phone swapped out - although most of these are probably old enough they’re over 3 years old and won’t qualify. A little unethical maybe, but Apple has had these problems enough it isn’t something I would feel too bad about since they drag it out until they only legally have to fix phones people stood their ground on.
The problem with trying to fix the board is Apple isn’t being open about the chip at fault, so unless you’ve repaired these for someone to use again it isn’t something you may know how to fix. I’m suspecting it’s due to a bad baseband IC (Intel in the inferior AT&T/T-Mobile SKUs or Qualcomm in the factory unlocked/Verizon/Sprint* SKUs) and see if it can be reflowed or reballed. You cannot change it as it is paired to the baseband IC, so you have to reball it. The work required to recycle the old board is not worth it - just change the board if it’s a hardware flaw.
*Sprint phones are locked, unlike Verizon postpaid. They got an ODI exception Verizon did not.