OK for all of those on Synology (DSM7), here is how I fixed my problem:
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Download the latest Synology pkg from the Plex website: Media Server Downloads | Plex Media Server for Windows, Mac, Linux, FreeBSD and More
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Uninstall your Plex from Package Center ticking the first option “Uninstall only”.
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IMPORTANT - Install your downloaded Plex pkg via “MANUAL INSTALL”. While going through this process, you have the ability to claim your server. Enter your claim code that you can get from here: Claim | Plex (At first, I attempted to do the install via the Package center (join beta button). This did not work out for me. I did manage to claim the server but all my settings and libraries were lost. Furthermore, when I updated my Plex using the latest PKG that I downloaded, I lost access to the server again)
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Finish installation and launch Plex. You should have access to your full library and settings