I wrote to Fairphone in November 2019 about having two ringtones. A* replied:
Nov 7, 15:30 CET Dear Fairphoner, My name is A* from the support team. I'll be your case manager, a pleasure to e-meet you :) Thank you for supporting Fairphone and for reaching out. Unfortunately, this is not possible as you can only have one ringtone for the phone. It is not possible to set different ring tones for different SIMS, we apologize for the inconvenience. I believe this is not up to us but up to Android to provide this, thus I do not think this will be coming any time soon.
The Fairphone 3 had Android 9 at the time.-