Ok, Fairphone OS, still Android 6.
Since I guess a factory reset or re-install of the OS would have been among the next recommendations, you could as well upgrade to Android 7 (which was released by Fairphone in the meantime) and see whether that fixes the APN problem.
Once you have enough time and calm, make a backup of your important data (pictures, contacts, messages, etc., everything you don’t want to miss) and get an overview about possible hurdles here …
… then check the Updater App for the upgrade.