I have never been able to port through the NVR to access an ONVIF camera.
What I have done is plug my computer into the switch on the back of the NVR, then set my computer to an IP address on the right network for that switch.
That works to access all the cameras on the NVR's network switch
I wonder if the method that works for Hikvision NVR PoE-connected cameras would work for a Dahua NVR?
Essentially the changes for Hikvision to gain direct access to such cameras are -
Set the camera default gateway to the IP address of the NVR PoE interface.
Add a static route to the LAN gateway (usually the router) to direct traffic for the network that the cameras are on to the LAN interface IP address of the NVR.
Worth an experiment?