The Synology NAS range can be used to run the Plex Media Server. However the Plex service needs to be configured with permissions to read your media files. In my example I have a Shared Folder on the NAS called “tv”, and I want Plex to be able to read, index and play these files.
Open the Control Panel, click on Shared Folder, then click on the folder you want Plex to be able to see, in this example “tv”. Now click on Edit then Permissions, you should see a user called “Plex” or “PlexMediaServer”, tick the “Read/Write” option and then click Save.
You should then be able to Rescan Library Files in the Plex console and the items should appear. You may need to reboot your NAS for the changes to take effect. I however didn’t in my case.
Agree with your suggestions/explanation with (at least in my case) one clarification: Changes to permissions didn’t “stick” until I came in as the Admin and made them. I had to come in through Synology Assistant and then make the changes, as opposed to the way I ordinarily came in (i.e, through Quick Connect).