It depends a little bit on what kind of contract this is. If it is a "regular" cable tv contract, report a move and the contract should be ended 3 months from the day your announcement was received (as you already moved out of Germany).
However if it is a GigaTV Net contract - thus using the GigaTV Net streaming service on top of an internet line, I doubt this would be possible - as you could use this service from any internet line. Thus relocation would not mean that the contract cannot be fullfilled anymore and you could only cancel to the end of the contractual term.