This forum is not for Plex Home Theater (PHT), but for Plex Media Server (PMS)
The PHT that QNAP have in their AppStore, does NOT work with newer versions of PMS!
Instead, you need to look at Open-PHT, as well as take your questions there
To find a QPKG for Plex, there's a nice person on QNAP forums, that provide it. Look here
/T