If you followed the guide, the new user should have all the required privileges to access, read and write in the database. Can you try using the MySQL's command line utility to test that?
mysql -u dbmasteruser -p dbmasterdatabase
If you can access the database, that means that the user is properly configured.
you should be able to use that user when accessing through phpMyAdmin. Can you try to run this command?
mysql -u dbmasteruser -p
Once connected, please run the
show databases; command. Did you get any error?