Hello @Sjors_Ehv
Have you already tried power cycling the Fritzbox router?
If this doesn’t work, is your current setup still the same? …
No additional network equipment?
If so, would you try how it goes if you remove the Fritz!Repeater 2400 from the chain?
Just to get a clue.
If it turns out that the issue no longer exists when everything is connected directly to the router, you can think about other options: