Devices connected via WIFI are not able to find DLNA services outside the WIFI network

Connect your DLNA devices to the Orbi router, not the fritzbox. 

If the fritzbox has a router built in, you could change the Orby system from router mode to AP mode, LAN to LAN. 

Or use the DMZ on the fritzbox for the IP address the Orbi router gets from the modem and use the Orbi router in it's current router mode configuration.