Welcome to the forum.
I’m not sure this is a Fairphone issue as the FP3 comes with Android 10. What have you tried?
Enabling dual ringtones
The first thing you must do is enable dual ringtones. Android makes this very simple. Open up Settings, and then go to Sound & vibration. From within that screen, tap Phone ringtone. In the resulting window (Figure A), tap the On/Off slider for Set ringtone for each SIM card (this will only appear if you have two SIM cards inserted).