As of today, I need to use 3 OpenWRT devices as "Dumb AP" next to a Fritz!Box.
I tried to set up one of those and it's working, but I can't see the devices connected to the AP in the "Device List" of the fritzbox. I only see the AP as shown here:
Assuming "OpenWrt-PT" on lan4 is your OpenWrt AP, you should be able to connect to luci on https://172.168.178.2/ - it does not integrate into the Fritz!OS GUI.