Sounds like the tester tool returns a false-positive when the Realm and Login attributes are not being defined, I’ll clean up the tester tool so it’s less confusing.
Authentication and Realm issues most often stem from the settings in “LDAP Login Attribute” and optionally in the “LDAP Role Manager Attribute” setting.
The “LDAP Login Attribute” is the attribute used to determine who the logging-in user is. Whatever value the user enters for username, this value is checked against the LDAP directory attribute defined. If the match cannot find 1 and only 1 DN, the user will get a login fail notice. This is because the plugin is unable to determine a unique person. If you use an email attribute to map to username values, the email value must be one that matches 1 and only 1 person.
The “LDAP Role Manager Attribute” is 3 parts, older versions of the plugin didn’t document this well. The 3 parts are “{realm} {domain} {accesslevel}”.
The “{realm}” portion is always literally WP, which denotes WordPress Realm. The second is the literal value “__ALL__” for all domains of WordPress sites your managing logins for, or the FQDN value for specific domain. The “{accesslevel}” value is one of the standard WP access level, fixed words: administrator, author, editor, contributor, or subscriber.
A DN can have 1 or many “LDAP Role Manager Attribute”, the highest access level match is used. This way you can centrally manage access control to any level detail you need.
Hope that helps.