I have suspicion that provider has blocked plex.tv IPs, and that's why my friends' parents MI BOX is the only client that can't reach our remote server.
I don't have access to MI BOX myself and have to give simple instructions regarding the manual setup. However I couldn't find if there is such a setting for the Plex Android TV app (client, not server) on youtube or google.
If there is a setting, could someone please write path to it from the Plex Dashboard and make a screenshot too? Will really appreciate that!
Settings ->Advanced -> Manual Connections
Enable Manual Connections
Enter IP
Enter Port (if different the displayed 32400)
Image