Thanks for your hint, I tried this in a few attempts - but unfortunately no success.
But I've ascertained that after disconnecting from the car, the mobile network connection is not established again, mobile network symbol in the task bar shows a "X" - BUT connecting to the car again, without a restart of the phone, works! Normal function in rsap mode. After disconnecting from the car again there is still the same issue - no reconnect to mobile network.
I've also ascertained that when the phone is in disconnected mode without mobile network after disconnection from the car, and I start the rsap android installation app, reconnection to mobile network works without any problems - AFTER the app runs the socket test at the bottom of the screen (all checkmarks green).
Do you have an idea why this doesn't work when disconnecting from the car? May there are any problems with the handover of the sim data back to the phone?