I think you just need to SDK port to be accessible (8000 by default). You *might* need the RTSP port, but I think you don't. No port forwarding needed on the NVR side.
Yes, you might just add it by IP address (if it is a fixed IP that will not change over time).
And you will probably get many people here saying that you are nuts if you do it like that, without setting up a VPN between NVR and camera.