Note that unless T-Online has additional requirements here this doesn't need to be your home address but only a valid mail address through which you can be reached.
Original German text of the law: https://www.gesetze-im-internet.de/ddg/__5.html
Google translation to English: https://www-gesetze--im--internet-de.translate.goog/ddg/__5....
Note that unless T-Online has additional requirements here this doesn't need to be your home address but only a valid mail address through which you can be reached.