This is so odd!
And gloves off
First, find your Plex Token, and take a note of that
Ref: Finding an authentication token / X-Plex-Token | Plex Support
Then make an SSH connection to your QNAP
Ref: Qnap faq.....read me first!
In the terminal window, type:
curl http://127.0.0.1:32400/media/grabbers/devices?X-Plex-Token=<INSERT_TOKEN_HERE>
Then show us the output, and when doing so, make sure to HIDE your token!!!
After that, also in the terminal, type:
curl http://192.168.1.99/discover.json
And show us the output from that as well