Plex Documentation → Remote Access & Granting Library Access
→ Setting Up Remote Access
→ Troubleshooting Remote Access
Plex Forums: The Basics of Remote Access Troubleshooting
Remote access is never working.
When Plex initially says remote access is working, it is really just trying to enable it. When that process fails, usually after about 30 seconds, it returns to the “not available..” message.
For remote access to work, your router must support either UPnP or port forwarding.
Your other devices are most likely connecting via Plex Relay. The stream is tunneled through a server at Plex and id limited to 2 Mbps.
Monitor streaming via Plex Dashboard → Now Playing + Expanded View.
If the connection is Indirect, then it is using Plex Relay.
