There is a self-service form where they call you back to confirm. However, they had issues with entering my ONT's ID in the system.
I am also subscribed to extra support. It's 5 Euro per month extra, but you skip all the queues and get directly connected to someone who knows all the technical stuff. With them it's basically giving them your ONT ID over the phone and they immediately set it for you (and they stay on the line until it's confirmed on both sides that it works). I will just call them again after the FRITZ!Box 5690 XGS comes out.
I am also subscribed to extra support. It's 5 Euro per month extra, but you skip all the queues and get directly connected to someone who knows all the technical stuff. With them it's basically giving them your ONT ID over the phone and they immediately set it for you (and they stay on the line until it's confirmed on both sides that it works). I will just call them again after the FRITZ!Box 5690 XGS comes out.