I would be inclined to re-install firmware as nothing to lose:
https://forum.fairphone.com/t/installing-the-fairphone-2-from-scratch-in-case-of-failure-komplette-neuinstallation-des-fairphone-2-im-fehlerfall/46728
Just put an EE SIM into FP2 running Ubuntu Touch and no problem recognising it and similarly in another running Fairphone OS just now.