Port forwarding setup for security camera

Eleven is correct in that you cannot initiate a direct connection to your modem from the WAN side due to the xlat

You either have to setup a zero-trust tunnel via a service like Cloudflare, which requires you to have a device on your lan that runs a program that keeps a live connection with Cloudflare that can keep a connection alive with said service. It's a bit to setup but can be done, granted I'm not entirely sure your technical level. You could lookup up YouTube videos or tutorials on Zero-Trust.

I personally use a raspberry pie to connect back to my XVR through a tunnel. Then once connected to the raspberry pi I have access to parts of my network like my phone is on WiFi. You can then just connect using VLC or whatever media player you might want to install on your phone to grab the ONVIF video stream coming from the NVR. Some NVRs only support RTSP instead, and if that's the case it will be difficult to have playback capabilities.  Every security system is different, so your mileage may vary.

PSA: Please keep in mind that you should do your homework on zero-trust setups, one could accidently leave a wide open door to your network if you don't know what you're doing. I have a couple of unique things that I have setup that will only allow my cell to connect to that tunnel.

Some defaults can be country of origin, device program checks etc. You can pretty much lock things down to only one device having access by at least 3 degrees of uniqueness, if not more..