My WiiM was unable to detect my library and connect to my Plex server. It showed “Unable to discover library”
First of all, I had to make sure that Plex is using the actual internal IP instead of the Docker virtual network IP. For docker installations, Plex MUST use host mode in the docker network configuration.
After setting this configuration, WiiM still refuses to connect to my Plex server. After messaging back and forth with WiiM support, it appears that the WiiM Mini device is unable to access my Plex server.
After doing some research, it appears that WiiM Mini (possibly other devices too) only uses HTTP to connect to a Plex server and not HTTPS. To make this work, I had to turn my Secure Connections to "Preferred" instead of "Required". To set this configuration, go to Plex Settings -> Network -> Secure Connections where you can toggle either "Preferred" or "Required". This made my Plex Server accessible for my Wiim Mini device.
I am not a fan of this, this means that all of my Plex users are able to stream without a secure connection. Please add HTTPS support for Plex so I can change it back to “Required”.