Fritz 7530 + Firewalla Purple = WiFi continuous connect and disconnect

I have a Firewalla purple and recently noticed that when it is plugged into my Fritz 7530 router, which is also providing the WiFi AP, devices continuously connect and disconnect in a loop from the WiFi signal (connects for ~1sec, disconnects, then repeats).  If the Firewalla is unplugged from the Fritz box then everything works as expected with a stable connection.  Can anyone recommend how to fix this?