the flat owes each month
The contribution is charged on the flat, not on you and not on your devices. One flat owes 18.36 euro a month. Tell this what is actually registered at your door and it draws the months since you moved in, so a second account shows up as a widening band.
Figures from the Beitragsservice: 18.36 euro a month per flat since August 2021, 6.12 euro with the RF mark. An independent commission has recommended 18.64 euro from January 2027, which the federal states still have to confirm. This is a plain arithmetic check, not legal advice.