Thanks for your recommendations. They helped a lot!
But, there is definitely an issue here - at least on my phone, which I will try to describe next, and which I did not observe using SFOS 3.4.0.22.
I tried using only one SIM card in slot #1 or #2 and disabling the respective not active slot. The behaviour is the same. Once you disable the slot, the mobile connection is lost, and also waiting a few minutes neither manual searching the mobile connection does not help.
But what helps is to reboot after disabling one of the slots. This way also with two sim cards inserted it works, i.e. disabling one of the slots, then reboot. The mobile connection of the active slot works fine then.
If I enable both slots #1 and #2 no reboot is needed to make the cards work and enable calls.
As a side effect. Being able to disable one of the slots helps to resolve issues with bluetooth connection to my car, allowing to use phone as well as bluetooth audio (A2DP) as I reported here: XA 2 dual sim: Bluetooth Audio / Car not working if second simcard active with SFOS 3.4.x and previous
It seems also that for this case using your regular simcard in slot #2 allows to avoid some issues connecting to a car.