The last and current version (possibly earlier versions, was on iOS for a time) of Plex for Android will not playback synced content while offline. I can sync content fine, it shows as synced and appears to play just fine when on WIFI. Soon as I’m on mobile data or in airplane mode, any libraries with synced content disappear from my menu and become inaccessible. Once I’m back on ANY WIFI connection, synced content is available. So it’s the presence of any WIFI that makes it work, even if it’s credentialed and I’m not able to sign in, only appears to require the connection to be in place. I’ve cleared cache, uninstalled, re-installed, etc., no change in behavior.
To those monitoring: this is VERY frustrating as it’s the main reason I use Plex and bought a Plex Pass. I am now forced to abandon it and look to other solutions for offline viewing, which pretty much puts an end to me and Plex 
