There are many ways.
Syno supports NFS and CIFS.
As long as you create a dummy account on the Syno for the Ubuntu “plex” user to act as the proxy (you grant permission to “plex” on the Syno like any normal user) then all is good.
NFS will let you use it under your username and Plex to use it under its username concurrently. CIFS is more restrictive.
These are all in the Linux Tips section
NFS
https://forums.plex.tv/t/how-to-mount-media-from-your-nas-shares-on-linux-using-nfs/197947
CIFS
https://forums.plex.tv/t/how-to-mount-media-from-your-nas-shares-on-linux-using-cifs-smb/197957
