According to this, I can "register" the new Modem ID by calling the helpline. With a little help from the conversation translator function on Google Translate and a little luck being directed to an agent who spoke english, I was successfully able to register my new modem device.

My question however, is if calling this number is the only way? The call tree to get to the agent itself is in German, and uses phrase recognition to select options instead of numbers. 

Can I do this via an email or a support chat session instead? Atleast this way I can use a translator more conviniently and as an added bonus avoid any mistakes in passing along the Modem ID as well. Or is there even (or can there be) an online form on my telekom.de contract portal to do the same?

The answer probably is that calling this number is the only option for now. If so, it would be great if the above can be taken as feedback.