Just write a cancellation letter in German language and send it to the appropriate address stating that you want to cancel the GigaTV subscription. As long as you don't mention your Internet contract - or write something like "I want to cancel all services" - your internet service keeps untouched.
But please note that it's not 84 days notice period, but 84 days to the end of the contractual term!
This usually means that you'll have a much longer period until your cancellation gets active.