Why can't I access plex on an external network?

My plex server can be accessed from home and all external networks, except for one. I am able to access plex from all of my friends networks, but plex does not work on my parents network. What could be the cause for this one particular network not having access? Possible solutions?

Most often it is the DNS server their network is using. By default, it is a server of their ISP and some have trouble resolving hosts on the .direct TLD. Which is what Plex uses to address the servers.
Take a look into the settings of your parents’ router. Search for an input field for the DNS server. If you find one, put 8.8.8.8 into it.
(This is the public DNS service of Google)

Then restart first the router
And after a few minutes all the Plex client devices.