Hi @Rose_Languste,
Thanks for your support. I eventually made it work!!!
It works even w/o resistors. I assume the Opener was not properly configured by Nuki. Wiring red and yellow to the source of the gong and black to the ground works like a charme (purple serves as ground for the intercom).
I couldn’t get the regular gong working by connecting green back to the bell. But I just left it open and I can live without the regular gong just with the Opener ringing