The Plex app on my QNAP TS-453Be gives a This site can’t be reached error

Plex Media Server NAS & Devices
2

Here’s a good article on how to connect to your server.
It looks like you just need to use the LAN IP for now. The initial setup isn’t too bad if you search the Plex website for articles, and have a look through the QNAP on Plex FAQ.