I've no experience of the 7104, sorry.
Does it have the 'Virtual Host' option like the 76xx/78xx series?
If so, this implicitly enables the 'kernel IP forwarding' facility between the NVR LAN and PoE interfaces.
All you'd need after that would be to add a static router to your LAN gateway (presumably your router), something like
'For network <NVR_PoE_Interface> route via 'NVR_LAN_Interface_address>
By the way - I've not (yet) tried the apparent SSH access to 7604 using 3.4.62, the observation was just from looking at the firmware contents.
This is something I'll get round to, that playback timeline feature is worth having.