The Fritzbox will be detecting radar on the DFS channels as is (was) required by regulation to prevent residential 5GHz WiFi interfering with radar.
AIUI OFCOM have relaxed the 5GHz DFS channels now but I'm yet to see any routers implement the changes.
Its only relaxed on band C which very few clients support as it required the end user buy a license from OFCOM to use before.
I had this argument with Zyxel as their AP doesn't support it as they claimed "most clients do not" but newer clients (Windows 11 with WiFi 6/6e cards) do which to me is the whole point of going WiFi 6, to use WiFi 6 clients. The issue I believe is they have to pay to have their devices certified again if they enable new channels, to make sure they don't create undue interference outside their spectrum.
I mean you'd think they would do that as a matter of course for the entire band the chipset can handle, but it seems not.