Follow-up: I found a post on another forum saying virtual host doesn't work with ONVIF.
My non-Hik cameras are connected as ONVIF and work OK via Virtual Host.
However - when you mentioned Virtual Host I made the assumption we were discussing access to cameras connected on NVR PoE ports, normally not directly accessible, which is what Virtual Host aims to provide connectivity to.
But it seems that's not correct, it sounds like you have set up an isolated subnet on the second LAN interface of the NVR and connected a set of cameras to it.
On the assumption that the Virtual Host in the LTS firmware enables the Linux kernel 'IP_forward' capability between the 2 NVR LAN interfaces, if you want to try it there may be another possibility.
My router is locked down by the ISP, so I can't use it to talk to a second subne
But, it seems, not via your router.
It's possible to set up additional routing in Windows.
So maybe try this out:
Let's say NVRLAN1_IP is the IP address of the NVR LAN interface on the segment to which your PC is connected, and NVRLAN2_IP is the IP address of the NVR second LAN interface. And LAN2 is the network address consisting of NVRLAN2_IP but with a '0' instead as the last number.
And that CAM2_IP is the IP address of a camera connected to the segment that NVRLAN2_IP is on.
At a command prompt in Windows (using Run as Administrator) first confirm there is no access to CAM2_IP
ping CAM2_IP
tracert CAM2_IP
Then try adding a route:
route add LAN2 mask 255.255.255.0 NVRLAN1_IP metric 2
And if that is 'OK!' then try the ping and tracert again.
If it does work - maybe optimistic but easy to test - then add a '-p' to the 'route add' command to make the route addition persistent.
In the absence of being able to add the route to the router, this, if it works, would need to be done for each PC used for access.
*edit*
tried setting it to the NVR's NIC address, which didn't help.
I forgot to mention that you'd also need to do this - the default gateway on the camera would need to be set to NVRLAN2_IP