Ok, I've found my solution. I need to change the port from ldap's 389 to ldaps' 636 because we use TLS protocol. And I also has to use method
NewLDAPSSLConnection instead to establish an implicit TLS connection.
I'm not sure why
NewLDAPTSLConnection does not work. It use explicit TLS which require us to establish a normal connection first (which use port 389) and then attempt to start TLS over that connection.