Explanation/Reference:
The DBaaS services are run under the Oracle Compute Cloud (IaaS). This has it's own firewall configuration, allowing you to limit access to your services. By default, all endpoints except SSH are disabled. There are a number of predefined "Security Rules" to open up the assorted endpoints, but they typically open the endpoints to public, which is rather risky. Instead, you should define custom rules, opening access to ports from specific machines.
Navigate to the main "Oracle Database Cloud Service".
Click on the hamburger next to the service of interest.
Click the "Access Rules" option on the popup menu.
Click the "Create Rule" button.
Enter a "Rule Name".
Select "" as the "Source" and enter your IP address in resulting box.
Select "DB" as the "Destination".
Enter "1521" as the "Destination Port(s)".
Leave "TCP" as the "Protocol".
Click the "Create" button.
You should now be able to connect to the database from the specified IP address.
References: https://oracle-base.com/articles/vm/oracle-cloud-database-as-a-service-dbaas-create-service