According to this, it seems to be a question of using the right adapter, and using /e/OS based on Android 9 didn’t seem to work.
As /e/OS on the Fairphone 3/3+ can currently be Android 9, 10 or 11 depending on how it was installed, what exactly does it say for you in Settings - About phone - Android version (tap on it) - /e/OS version?
What about the Wi-Fi hotspot feature of the phone or USB tethering?