Don't know how the cable version works.
But it should be always the same.
System - FritzBox User - Create a user and add "Internet Access".
Internet - MyFritz!-Konto / Account: Activate "Internet Access", use the Letsencrypt certificate.
Then you have a random address
with a random port.
PS: Because it's a myfritz.net address, Fritz is able to create a Letsencrypt certificate and should renew it.