I can’t help you with the optimum router portion. But I can help you with the port forwarding part. The short version: You can’t use UPnP to automatically set it up. You’ll need to do manual port forwarding.
First, here’s a primer on manual port forwarding:
https://support.plex.tv/articles/200931138-troubleshooting-remote-access/
On your plex box, you’ll be using port number 32400. But on the router, it can technically be any port number you want, as long as you set that same external port number in the remote access settings on your server. But for these purposes, let’s pretend you’re using 32400 everywhere. (These should all be static IP’s.)
Optimum router WAN IP (on the internet): 300.300.300.300
Optimum router LAN IP: 192.168.0.1
Your personal router WAN IP (to the optimum network): 192.168.0.20
Your internal router IP: 10.0.0.1
Your Plex server IP: 10.0.0.2
First you need to set up a manual port on your optimum router (I don’t know how you’d do that). Set it for 32400, TCP, and forward it to the WAN IP of your internal router (192.168.0.20 in the example above).
Next you need to set up manual port-forwarding on your personal router. Set it for 32400 as well, TCP, and forward it to the static IP of your internal Plex server (10.0.0.2 in the example above).
Once that’s done, you might restart everything to be safe, but honestly you shouldn’t need to. The path should now look like this:
Internet -> 300.300.300.300 (32400) -> 192.168.0.20 (32400) -> 10.0.0.2 (32400)
That should take the Plex traffic all the way into both networks and to your Plex server.
