I think you may be out of luck. Best solution I could think involves a tunnel/VPN from one location to the next, so you would instead just have the camera using a private IP address and enter that private address in the NVR, this would also be more secure than having your camera directly exposed to incoming internet traffic.

With the sounds of your current setup, keep in mind that if the IP address changes when something important is happening that needs to be recorded by the NVR, you may be missing out on some footage and you need to test how long and if the NVR system can recover gracefully when this situation happens. You say that a VPN or static IP address isn't an option, but I find that hard to believe, every ISP I've worked with was always willing to give out static addresses for business accounts and VPNs are pretty easy and cost effective to setup, a lot better than having internet facing cameras being compromised.

As I was typing the above, I thought of one other possible solution, you could write a script to monitor the dynamic domain (ex:camera.dyndns.org) and detect when the address changes, when it does detect the change, the script will login to the NVR and manually change the IP address. Again though this could still cause blimps in the video footage and some gaps in footage.