The Fairphone works in 4G, so that’s good.
Is your country and operator already listes in the VoLTE test results? VoLTE provider support
So does the phones stay on 3G always? Or just during calls? The phones should idle in 4G network no matter what, as long as you gave “prefer 4G” set in the settings. VoLTE requires flipping the switch on for both devices.
Sometimes the initial 4G requires aome time, so juat leaving the phone be overnight could help… Or flipping the flight mode on and off.
Do you have mobile data enabled and functional? That shouldn’t be related, but it’s in the same ballpark anyway…
One more obvious thing is coverage for 4G.