Synology Directory Server - not to be confused with Synology LDAP Server - is a Samba-based installable package for Synology. NADI has no official support for Samba but it has been tested successfully against Samba 4.12.14.
Setting up Synology Directory Server
Set up your domain as described in the official documentation.
When connecting NADI to Samba/SDS, you have to choose STARTTLS for the configuration option Environment/Use Encryption, otherwise you will receive the error
AD: Strong(er) authentication required] [AD error code: 8 during verification of the connection].