RUT955's GPS NMEA functionality simply sends RAW NMEA information to specified server. It does not have any authentication.
Below you will find brief example instructions how NMEA data could be sent to TCP server on computer. Screenshots in example are made on slightly older WebUI design, but nonetheless whole idea still remains the same with current latest firmware version.
Instructions (4rth page is not relevant for you in this situation): https://community.teltonika-networks.com/?qa=blob&qa_blobid=15489023894454431801
Exactly the same approach can be done on your Rasberry Pi: you would simply need to launch simple TCP server to capture the data. Afterwards you would be able to use any other software to parse NMEA data.
For testing purposes you could start everything from sending data to Hercules TCP server application, running on computer.