Plex Outside the Network
I have a Century Link Gateway that I put in DMZ Hosting Mode. I have the run the Eero in Double NAT since Transparent Bridge Mode is not possible. Upon testing Plex I can't get outside the network on the Ports I have designated both on the Gateway and Eero.
Double NAT is not good for Plex. At this point the only solution is the have the Eero run in Bridge Mode, but nearly all the cool features like limiting my 15 year old daughter's internet goes away. I would like to have both, but will sacrifice giving up the Plex outside the network for the features. Any ideas I am not thinking of?
With the caveat that I have none of this hardware myself, so this might not be helpful....
What device are you running Plex on? I'm a bit surprised by your statement that the software is affected by double NAT. As long as all devices that are inter-communicating within the home are running from the Eero network (wired or wifi) that part of the equation should be OK.
But you do need to make sure that the two networks (Century & Eero) use/generate different IP ranges. I'd suggest, to make it completely clear, that one be on a 192.168.x.x range and the other on a 10.0.x.x.
If the Eero is connected to a DMZ address on the Century (which is a substitute for explicit port forwarding rules) you'd just then need port forwarding configured in the Eero to send incoming Plex connections to your Plex server. You'll probably need to have that server configured on the Eero network to have a static IP address.
I believe you are on to something as all the IP addresses are 192.168.x.x. I will have to do some digging to find out how I can get the Eero to to assign the IP address. Also my Plex server resides on my iMac, which I think complicates things because the IP address has changed a few times.
It is possible to have the two networks under 192.168.x.x but they need to have their subnet range (mask) defined such that they don’t clash. eg. 192.168.22.x / 255.255.255.0 and 192.168.33.x / 255.255.255.0
The iMac really needs to be assigned a static IP, either within its own network preferences or via MAC DHCP reservation on the Eero..... otherwise the port forwarding can’t work.