At first you need to select Directory Server in Package Center to install it.
Click on Install if not installed.
Next, click on Directory Server icon.
Set your FQDN and password (eg: ldapsrv.local) and press on Apply button.
You can manage Connection Settings later, but for tests, don't set it if you want.
Next, manage any users and affect them in users group.
Setting User Manager / Directory / LDAP server.
Create a LDAP server and select: OpenLDAP Directory.
After submitting, FreePBX should be connected to the directory Server on your NAS Synology.
Next, Edit any groups and allow any privileges like :
- Administration Access ...etc