Plex Web App Not Connecting

[Dirk Diggler]

I can't get remote access to my Plex server using the web interface. I get remote access on my apple and android devices, but not via the web app. I get this message in the Plex settings section:

You have successfully signed your server in to Plex, but we were unable to reach it from outside your network.

I've Googled for hours and tried to set up port forwarding etc but I must be doing something wrong. I'm using a Virgin Superhub 2 on 100MB internet.

I don't understand why I need to set up port forwarding when I can get remote access on android and apple devices?