for configuring tvheadend with a Fritzbox 6660 Cable
first make sure Live-TV is enabled on the FB:
!Screenshot_2022-02-06_10-30-29.png!
Tvheadend should find the device (or is there something else to setup?):
!Screenshot_2022-02-06_10-33-08.png!
The Adapter and each Tuner is configured like this:
There's no need for dedicated IP addresses for each tuner.
!Screenshot_2022-02-06_10-33-29.png!
!Screenshot_2022-02-06_10-34-06.png!
Hopefully this will help.