You will need to configure these three items on your on-premises server and do a restart of your Ragic service:
LDAP_AUTH_SERVER=ldaps://your.server.name
LDAP_AUTH_DOMAIN=your.active.directory.domain
LDAP_AUTH_EMAIL_DOMAIN=your.email.domain
LDAP_AUTH_SERVER is the domain name or IP address of the LDAP service like Active Directory that you're connecting to. LDAP_AUTH_DOMAIN is the domain name for the principal on the LDAP server, and LDAP_AUTH_EMAIL_DOMAIN is the domain name part of the email addresses that the users will be used to log in to Ragic.