Do you regularly backup data upon your FP2 as here? https://support.fairphone.com/hc/en-us/articles/210512123-How-to-back-up-your-Fairphone-2-and-restore-the-data
If you have excluded SIM as a problem, you would need to exclude “software issue” upon device which would then point to “hardware issue” upon motherboard SIM holder unfortunately. As price of pre-owned FP2 now falling due to imminent delivery of FP3 I doubt it is viable to consider replacement of motherboard now, especially as device would have to be sent back to Fairphone for repair.
I appreciate that thought of re-installing device firmware maybe outside your comfort zone, but don’t know whether there is a “Fairphone angel” in your part of the world, whom could help you out: https://forum.fairphone.com/t/the-fairphone-angels-program-local-support-by-community-members/33058