Technically the NVR should work even if the cameras are on the external internet, the issue might be on how much can be done automatically. For example, when you first plug a Dahua camera into the NVR it does a bunch of setup on it automatically, like setting a default password. So each camera will likely have to be setup with its web interface.

I’m pretty sure I’ve read somewhere that you cannot have more than one camera plugged into a single one of the NVRs private network ports, so all your camera traffic is going to have to be routed through the main network port.