I have not tried it as an access point. In my case, the box is in operation as a repeater on the Sunrise box. I currently have 2 units in operation like this and without interference. Greetings Stephan

I operate the repeater as an access point. This requires a connection with an Ethernet cable to the network, in your case to the Sunrise box. I recommend that you ask Sunrise what is recommended. I am not entirely convinced by the Fritz product. It ticks around and doesn't want to accept any settings from the "control centre". However, it can be set manually, which is necessary for you.
For months, AVM has been touting an update of the operating software, but it is still not available for me. I bought it because I already have a Fritz router.