the database handler was found with $c->app->_dbh_db();

The underscore often implies that the developer of the module meant these as 'private' (or internal) functions (and therefor may not be documented). I believe that that is also the case here. I looked at the source and there is definitely a helper created by the name 'db'. Try to use the method from the previous link that I gave you: A Simple Mojolicious/DBI Example