If you’re a 453be the package you want is the x86 (since it has an Intel CPU).
You can browse the app store and click on the Plex app manually:
https://www.qnap.com/en/app_center/?middle_place=1&qts=4.4.1&type_choose=Entertainment
Stop Plex. Install the package manually via the QTS appcenter. Start Plex.
