Please see https://www.davx5.com/tested-with/synology:
The Calendar package seems to provide a CardDAV service, too. This is why “CardDAV” appears in DAVx⁵ accounts created for Synology Calendar. You can even create an address book using DAVx⁵. However, this feature is not officially supported and there’s no user interface in the Calendar package. So, use the CardDAV server package instead (see below).