Ok, Plex is a server-client system, so the Plex software you've installed communicates with a Plex server to tell it what you have. You seem to be trying to use it in a way that VLC or Kodi clients...

There's a lot of potential configuration issues, from firewall (on your NAS) to Plex server setup. I'd err more towards asking on a Plex forum as it's unlikely to be an issue with the router itself,...

@Bolton-UK I've just manually updated mine (V3 EU, in the UK) and they seem to be alright so far. I used the web interface.

@Mycroft I asked as I recall an issue with Dlink (I think) switches when communicating with TPLink devices, but obviously not here. Is the Synology a single NIC device? If not, does it perform the...

@crombinator there are logs in the admin area webpage on the deco. Browse to your main Deco's IP in a web browser (log in with your TPLink password) Go to Advanced -> System -> System Log

@Mycroft what switch do you have?