Look here for documentation: https://docs.mendix.com/appstore/connectors/database-connector/
And here for the module: https://marketplace.mendix.com/link/component/2888
The external database is not visible in the Domain Model. Consider https://www.mendix.com/data-hub/ if you need Data Integration for Citizen Developers.
Go Make It
Follow this link to connect with External database(MySql):
This may help you!