Apparently the problem is Android + PMS 1.18.
WD claims that Plex has to update their software in wd’s store, but at the same time WD doesn’t let their users to update it manually.
If you search for “plex 1.18 android” in this forum you’ll see a bunch of topics like yours in the last week. Plex is aware of the problem
About recommendation, I would go with Synology, but Plex has a document with NAS models and their capabilities. ChuckPa is the person who can provide this information while it’s not publicly available.
