Unfortunately, you would not be able to accomplish your desired solution with RUT500. This is related with aspects, that normally both router's LAN (connection to cameras in your case) and WAN (your WiFi WAN connection) interfaces cannot be using same network. This is a general rule of networking, since having same network accessible through multiple interfaces causes networking issues.
Of course, to each rules there always an exception and particular for such solution "relayd" OpenWRT functionality was created. More information about is can be found here. Our other devices, e.g. RUT950, etc have this Relayd functionality, however it is not implemented on RUT500, therefore you cannot have both LAN and WAN networks using same network.
In your case, maybe the following plausible solution would be an option: connecting RUT500 with default configuration (LAN network 192.168.1.0/24) to your NVR's network and using port forwarding functionality, so that request from RUT500's WAN would be forwarded to your cameras? That might or might not work, depending on your NVR aspects. Apart from this suggestion there are not a lot else what could be done, since routers in general are meant to connect multiple networks together, and not just extend existing one. If you need more physical LAN ports on your NVR for your cameras, you could use switch instead of a router.