Lumin L1 runs a UPnP media server, but does not export the music via a network file system. Therefore Roon Core cannot see it via network. (L1 is meant to be a zero-configuration alternative to NAS, so it has no extra features beyond what is needed to support Lumin app).
You may connect L1 to the Roon Core PC as a USB HDD, then Roon Core can access the music files.