Hey burns, I have searched for a post like yours, actually, my first MX is on the way (for my homelab)
It is possible!
As I want to replace the current setup with an MX, I will keep the Zyzel/Digitalisierungsbox Modem. It works flawlessly. All you need to to is to change the line ID, call the Telekom support for rediscovery and that’s it.
Telekom contact for rediscovery/site for the order process: