Scrap and start over. Most likely, they will be looking to you for support and you don't want to support the crap that's there. I've been down that road before. It's not worth it.
Advanced IP Scanner - Download Free Network Scanner. for your PC
Fing for your smartphone.
What does your router say for DHCP range?
On your PC, open a command prompt and type IP config. Tell me what you have for your IP address and Gateway.
I would focus on getting everything working locally first. First, make sure you are in fact on the same subnet (192.168.0.x). Make sure the .200 is outside the DHCP range in the router. Don't worry about the IP addresses of the cameras. You are correct that the NVR handles that. I'm assuming the...
Ok, I just uninstalled OpenVPN from my Untangle server and reinstalled it; generated new OpenVPN files and I'm good to go. So your assumption on the Netgear may be correct. Hopefully, someone who knows Netgear will chime in.
That's indoor cable. He needs outdoor cable. I've used this: Southwire 500 ft. Tan 24/4 CAT5e Indoor/Outdoor Cable-56917645 - The Home Depot
CAT5e is fine for a short run.
You could also use flexible conduit if your environment is really extreme.