My Republic + Static IP + Plex = Double Nat? :(

Hey sorry if this is the wrong forum - I wasn't sure it would be an ISP issue so I didn't post in the My Republic forums.

So, some background. I had everything working perfect until we moved addresses - however literally all I did was move my server cupboard to a new house and plug my AMPLIFI into the Huawei ONT here. Inside of my network everything worked seamlessly when we moved but I've lost outside access.

My setup is My Republic with a static IP (I pay for) into the AMPLIFI to my server which is connected via ethernet on a static IP.

I have the ports forwarded via the AMPLIFI config and the same port as my "Manually specify public port" in Plex setup.

Edit - so the error I'm seeing in Plex is:

Your server is signed in to Plex, but is not reachable from outside your network. Learn more Tip: It looks like your server may be connected to the internet through multiple routers or other network devices. Try connecting it directly to your primary device, or visit our support site for more information about troubleshooting this "Double-NAT" scenario. Learn more

I can see my static IP using the various online checks and Plex shows the correct IP address as my public address.

I've tried -

changing the port on everything

Disabling the firewall in Windows

Got a friend to try from their home

Updating router firmware / resetting etc

Contacted My Republic to confirm I'm on static IP

Using "Can You See Me" the port check results in a time out

The only odd thing I have noticed (and I'm not sure if this was the same before we moved - because it worked I never checked). My IP address under the 'Internet Tab' in the AMPLIFI config is different from my static IP.

I assume this is correct and My Republic would forward all the traffic from my static IP to my "WAN IP" for the lack of a better understanding?

Any ideas? My wife is away at the moment and not pleased she can't watch her TV shows :(