The Credentials that should work for your application sign in is indeed the MxAdmin user you configure in your Mendix Service Console. Start up the console, go to the Advanced menu, and look for the option to 'create or update administrator.' This will let you set the password for MxAdmin. Keep in mind that most databases used in on-prem deployments are case sensitive.
If that password doesn't work for you, you can see why in the logs. If you look at the application logs, as output to your /logs folder you configured during setup, you will see a message for the failed login. Perhaps it is 'User could not be found' which means maybe the case is wrong or 'incorrect password' which means you should try resetting the password again.