Referring to your other recent question about screws and putting a FP3 back together, I would begin by making sure the phone’s components are properly assembled and secured, with all internal contacts working properly. See here for more info.
I think a good start would be to clean all internal contacts with 90° surgical alcohol or isopropanol and some cotton buds. Leave to dry for three hours before reassembling the phone.
Did you manage to find spare screws? A high-quality electronics hardware store might be of help.
The phone’s components must be properly installed before looking anywhere else for a solution.