Question about AVM FRITZ!Box 7490

I can't help you there. Sunrise has obviously had better experiences with the Sagecom part as far as DSL is concerned. I also have many connection interruptions and Sunrise's solution suggestion is to switch to Sagecom for me too.

I am not with Sunrise, but I phoned them a year ago to find out. At the time, they said that only their own Fritzboxes (supplied by Sunrise) worked and were supported.
Of course you can (like user Bire309turbo) tinker with something if you can get hold of the credentials, but if something stops working, you are dependent on yourself...

Sunrise supplies its own device - a Fritzbox if desired. Of course you can integrate a second Fritzbox into your network. In my opinion, using your own device instead of the one supplied by Sunrise only fails because you cannot get hold of the credentials and Sunrise will not hand them over.