Searching/no service after screenreplacement

I had a customer that repaired a iPhone6 screen.

Everything worked fine.

They came back after 1 month, having no service or its just searching.

After i replaced the charge port the i6 got service, i handed out then phone to the customer.

Got a call after 10 minutes and they got the same problem again.

I have tried everything following:

Changed chargeport.

Reset network settings.

Reset Phone.

Tried different simcards.

Checked for water damage. (There were none)

Cleaned the motherboard with ISO.

Date and time is correct.

Correct country is set.

Anyone had any problem like this before and know how to fix this?

// Worried Service technician.