Since Android 10 update I got yesterday my dual/hybrid sim S10+ is now accepting remote sim access from my car (2015 Skoda Superb II - Columbus RNS510 head unit) Bluetooth premium is now working giving me SMS control again
Hope this is finally sorted for all, regardless of car manufacturer.
I think I read somewhere that there was 5 conditions to make it work:
1) Sim needs to be in sim slot 1
2) No sim in slot 2 (not sure if this is true)
3) Delete your bluetooth phone in your car bluetooth settings
4) Delete your car from your phone bluetooth settings
5) Enable RSAP in car (premium phone)
6) Pair your phone with car, now it should work
On my S10 it now works (also make sure rsap (premium phone) is enabled in the car
In my skoda I also needed to enable "roaming" and "auto connect to data"