Make sure the Plex client (PC, Mac, etc) is on the same network as the NAS - 192.168.100.x - at least for the initial setup.
Additional comments:
Do not put any of your media in the Plex shared folder. This will cause problems.
Do not update to DSM 7. There are major issues with Plex and DSM 7.
Review the following documents. They have a lot of very helpful information.
Installation & Setup of Plex Media Server on Synology
Synology FAQ (DSM 5 & 6) - Questions, Answers, and a few How-To’s
