@Tedd404 I only have a Port-Forwarding on UDP1194 from FritzBox to ER707 - it worked pretty well until now, thats why I'm little bit confused :-)

Hello everyone, I am using a FritzBOX 6690 as a modem (with IPv4 address, no DS Lite), my ER707 is behind it (no exposed host), port forwarding in the FritzBox is set. Only the router is connected to...