Hub 2 no longer allowing Plex connections

[#1]

Hi. I run a Plex server off an nVidia Shield box. It all worked fine with my previous model of router: I'm not sure what that was, exactly, but the last time I renewed my contract I was told it was very old and they sent me a Hub 2 instead.

The Hub 2 is fine, but it seems to have stopped Plex from working properly. The web interface now says "app.plex.tv is unable to connect to "localhost" securely" *unless* I connect through the Shield's IP address rather than the app.plex.tv address. When I do so, I can get into settings, and under "Remote access" it says the server is "not available outside your network".

Had a quick look at the troubleshooting guide linked by Plex and go out of my depth very quickly. I got into the admit settings on the router and tried turning UPnP off, which did nothing, then back on again, which still did nothing. The next section of the help guide wants me to setup manual port forwarding, and I'm getting concerned that if I dabble too hard, other things will stop working.

I'm using a WiFi mesh system plugged into the router, which may be part of the problem. The plex settings page says "your server may be connected to the internet through multiple routers or other network devices". That said, the server itself only has the connection details to the mesh system, not the signal of the router itself.

Can anyone help me with this?

[#2]

I suggest that you need to 'layer' the issue.

You've presumed that the issue is the Hub2 and the TV, but you've got a complex network.  Can you eliminate the mesh and get the TV working with the Hub2 alone?

If that works, then it suggests that the issue might be between the mesh and the Hub2 ... with would be a different matter.

[#3]

Thinking on from @Townman comments

How is the Plex Server network interface configured, using DHCP or manually. 

Not seeing the outside network could suggest a default gateway issue, where the plex server sees no route to the outside network.

[#4]

@Dan_the_Van 

Very good call.

We all tend to make the mistake that things are configured “normally” in a user’s set up with no manual funny business … not mentioned by the user.

Manual configuration of the TV relative to the old router wound make a right dog’s breakfast of things with a different router working on some other LAN network address.