Fire TV can't find remote server but iOS devices can

Yesterday, at home I installed Plex on all my devices. Amazing, just all worked. Today, away from home where I have a new fire TV 4K, I installed the Plex app. After signing in with the TV link code via my iPad, the Fire TV says registered but then ‘can’t connect to myserver x.x.x.x’. Now my iPad and iPhone Plex app work fine remotely, so remote access is clearly possible. The x.x.x.x shown on the Fire TV error is the iPad address of the server on a local network at home though, so I’d expect this not to work. I’ve read through the Plex advice and restarted etc to no avail. Am I doing something wrong?
Thanks for any pointers.