Dear Marcel
with your configuration suggestion "Please connect the red opener cable to the positive terminal a1 and the black one to the GND b2) I managed to get it working.
I needed to turn off bluetooth and WIfi, otherwise I always got a timeout in the step where Nuki tried to connect when going downstairs to the main entrance of the building. Once I did everything over LTE vie the bridge it worked.
However ring to open does not work. Essentially in the configuration, I needed to lie when asked whether ringing lead to opening the door, otherwise the configurator only exits with “not compatible”.
Once I clicked yes, essentially I lied, the configuration ends with success. I can now open the main entrance via the Nuki app. RIng to open does not work.
Nuki needs to reprogram the configurator to take these steps and the results into account. You would have more customers and/or make more customers happy that way.
Anyway, I am grateful to @Marcel_Urban for his advice. Maybe other customers can use the info provided through my experiences.