Do you know what version of Plex was installed on the NAS?
Do you have an existing server on the NAS you want to save, or do you want to start from scratch?
If the former, is the Plex Data Folder (database, etc), in the /plex or /plexmediaserver shared folder?
The reason for the questions is that Synology forced a lot of changes between DSM 6 and DSM 7.
There is one procedure of you had a working server on DSM 6 and it no longer works after you moved to DSM 7. If the /Plex shared folder exists and contains the database, then it is possible to recover the old server. See FAQ 27 - Migrating an existing PMS from DSM 6 to DSM 7. You need to give system internal user PlexMediaServer full control of the /Plex shared folder and at least read only access to the media files.
If the Plex server existed only on a DSM 7 system, then it is a different procedure. See FAQ 29 - DSM 7 - Give PlexMediaServer access to your media.
