No doubt you have already tried resetting device to factory settings with new SIM in place?
Which of the two SIM holders do these FP2 have?
https://forum.fairphone.com/t/fp2-microsim-adapter-broke-sim-card-slot-pin/12225/26?u=aspergerguy
With older design on the right it is too easy to rip off concealed spring contact when removing SIM if nanoSIM used in holder to make microSIM size.