Without digging too deep into the steps you followed because I’m a bit lazy atm, I can offer up two sources of information that you may not have read:
https://support.plex.tv/articles/201370363-move-an-install-to-another-system/
and the Synology FAQ:
It sounds like you are close. There are a few more articles on the Plex website if you’re still having problems. I won’t paste them yet.
Did anything in there help?
