Thanks for the help all, I worked out that my IP was floating, changing every so often, so I made it static which fixed the issue. The synching was referring to the media library updating on other devices with Infuse installed, other Apple TV units.
For my point of view, depending of your use of TV. I don't recommend Plex for only one reason : energy use and NAS life.
Plex require to the NAS to be always awake and prevent sleep mode as it as a task to check folder and refresh the library often. Since the last update you can enable Wake On Lan on Infuse to start your NAS when it is in sleep mode. More over Synology are really good at energy saving if you use the good apps/tools, and Plex is one of energy killer. So if your library is not to big (less than 5/6To), I recommend to just use SMB. Enable SMB 3 (SMB 2 is the default) and put all options to put in sleep a soon as possible and your NAS will have long life!
I have a 4To library (all optimised in h.265, that's helpful) and my NAS only start when I use Infuse, 2/3h hours a day at night. I tried Plex for weeks and My NAS was always running and my CPU most of the time a 100% for no reason.
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