Plex is a very popular media server software that can organize and stream your own music library. WiiM supports the DLNA (Digital Living Network Alliance) based digital media server. Plex Media Server can function as a DLNA server in addition to its normal Plex functions. This allows the WiiM Home to access content from the Server and cast the music from Plex Server to the WiiM devices such as WiiM Mini.

Please check this article below to set up DLNA on Plex:

Some of the important setup steps are shown below - 

1. Enable the DLNA server on Plex

2. Add your music folder to libary on Plex

3. Open your WiiM Home App to access your music libary on Plex

Enjoy Plex + WiiM.