Hi and welcome to the community forum 
Sorry you lost your phone!
I presume that you’ve already tried changing cable and charger. Maybe you use the same ones for charging the other phone so you know that they function correctly.
I think I would start with the hardware. Remove dust and lint with the small brush tool of a vacuum cleaner, or compressed air, and a wooden or plastic toothpick. Pay special attention to the USB port. Disassemble the phone (see the Ifixit tear-down here). Make sure all the contacts of the modules, battery and battery compartment are clean. Use a cotton bud with Isopropanol or high-grade surgical alcohol. Allow to dry overnight before re-assembly.
If necessary, you can check whether the bottom module needs replacing. Swap that of your FP3 with that of the other phone (if your GF is willing of course!) If that’s not convenient, check out the Fairphone Angels page to see whether there’s one near you who might be able to help.
For the mobile data problem, you presumably have a new SIM after losing the old phone, but make sure it’s properly seated. You can swap it to test in the other phone. I would expect the APNs to be identical in the two phones.