Very good! Progress!
You could look at systemctl status ondemand-dex or journalctl -u ondemand-dex but I don’t believe it’ll say anything meaningful. That is, unless you’re unable to bind/connect to your LDAP at all. That could be one issue. That you’re unable to query LDAP at all. I don’t know how dex would respond to the user with this info.
If you can correctly bind - then I’m guessing this is the documentation you’re looking for. My guess is your search/filter/query isn’t quite right.
https://dexidp.io/docs/connectors/ldap/
There’s also this note that I left for myself around ldapsearch and different ways to debug you’re query. I get the feeling that you’ll need to experiment with ldapsearch to refine your settings.