Question about AVM FRITZ BOX 5690 PRO INT

No, it's not that easy. You have to be technically savvy and inform yourself in the forums to get the box up and running. Sunrise does not yet officially support third-party boxes, so you have to rely on help in the forums. There is no guarantee that it will work. With older boxes like the 5530 it usually works, whether the 5690 works is written in the stars. And telephony is another topic, with a lot of luck that will also work... most of the time.

Among other things, the AVM router can be connected to the fibre optic network without any problems. The prerequisite is that the provider (e.g. Sunrise) agrees to this.

Yes, this works without any problems, two different adapters are also included.

I can't answer this question because I don't have a fibre optic connection.