Your QNAP is sadly not supported by Plex, since we only support QNAP OS QTS 4.3 and newer
Ref: ARMv5, PowerPC, and ReadyNAS ROS4 no longer supported starting with Plex Media Server 0.9.17.0
You might be able to find the latest supported version ( 1.14.1.5488 ) in QNAP AppCenter, but I doubt the current clients will work with such an old version
I suggest you use your QNAP as a NAS instead for your medias, and run the server elsewhere
