Hello everyone - thank you very much for your help.

Here is the solution:
In the scope of delivery is a 16v transformer where you can install in the fuse box. Simply remove the old 12v and insert the new one (with the fuse switched off, of course). I then only had to bridge the old chime (but this is due to my house installation). Doorbell installed and configured. It's important to make sure that you have really good WLAN at the front door :-)

I am pretty sure that the G4 Doorbell must be operated with 16V or 18V.
However, the scope of delivery includes a 230V to 16V/18V converter for the top-hat rail, which can be installed in the sub-distribution board.

However, the Chime can be operated via a normal socket.

DoorBell and Up-Chime are connected via WiFi and both can be managed in the CloudKey Gen2. DoorBell can be connected to the existing 12V. Connect Up-Chime to any socket.