Internet switches between Fritzbox 7530 and 6660 cable, any idea how to fix?

We own a Fritzbox router 7530, but since last week the internet interrupts every 30sec-1minute and switches to 6660 cable. Ive already contacted customer service and all they did was reset the router a couple times, send me a new router, and then again reset it a couple of times. I dont know anymore what to do.

My roommate who uses a LAN-Powerline adapter claimed he doesnt have this issue on his pc, but every phone and my pc connected to the WLAN has this issue. I borrow his LAN-Powerline and the issue still occurs for me. now i doubt that he never had this issue. 4 devices and 1 doesnt? makes no sense.

Anyway if i start counterstrike 2, it connects to steam servers fine initially, but after a minute it disconnects. This is constantly on every site. If i watch stream or movie, the buffering bridges it but i can see the issues happening in real time in twitch chat for example.

Does anyone know how to fix this?

I have reset the router countless times, i have split the internet in 2.4 and 5 GHz.

Another example, i open up the user interface of fritzbox, "Welcome to 7530: log in", i want to go to the reset tab aaaand it kicks me out. "Welcome to 6660 cable login:"

I cant because i dont have the PW to this fritzbox.....

Any idea on how to solve this issue?