I wrote this HOWTO, using LDAP on Synology so I could try out the Synology Directory Server. This is how I managed to get Linux machines to authenticate against it. At the time of writing, Synology was on DSM 6.2-23739 Update 2. I will be using Ubuntu 18.04 as the Linux clients
The Synology documentation
for Directory Server is appalling only stating the blindingly obvious. That is all assuming the page has been updated, most having missing options or features.