Supersonic - don't sign up

I have a month to month LTE contract with Supersonic which started middle May 2021 I informed them middle Nov 2021 that I was giving notice to cancel end Dec 2021. The call centre agent informed me that I would have to pay R250 to courier the router back, which I agreed to. Now I RECIEVE a "Clawback invoice for over R2800. On a month to month contract that is up to date. How can that be ??? I have instructed my bank not to honor the debit order. I suppose they will cut the service, that fine I already have a better service provider