Hello,
I have tested a similar set up, however, the device kept sending NMEA data to the configured LAN IP:port combination.
My RUT had two LAN devices, one was running Hercules to receive NMEA data, for testing purposes, the other had its interface configured to receive mobile IP. That is exactly the performance the router provided.
What is the firmware version your device uses?
Is your device receiving NMEA data the same device, which gets the public IP from RUT955?
Have you defined a MAC address of the interface in your LAN to have the public IP passed to?
Enabling bridge mode or passthrough disables most of the RUT functionalities in regards to that the device, which receives the public IP, is unable to access the router and that the router looses access to the internet.
Best regards,