Hi Remko,
Since you can see it in VLC port forwarding must work in the firewall at 85.1xx.
In the Custom URL make sure you set stream type to rtsp_tcp.
Make sure that the password do not contain special characters like %, &, … Only letters and numbers.
What camera is it?
-Henrik
Change to rtsp_udp if that is the problem.
There are many models. What´s the model number of the camera?
Since the camera support the ONVIF standard best way is to connect the camera on the same LAN as the computer. Add the camera to NCS with the tab for ONVIF Source. When it is running in NCS go to the tab for Custom URL where you have the correct URL for the camera. Just change the local IP of the camera to the public IP and it should work if all is open on the way to the camera.