Can't Connect to New RDS MySQL Instance

I need to create a new RDS MySQL database by August as my previous EC2 RDS instance is being retired. I last created this core database in early 2013 so admittedly I'm a bit rusty. I've created a new MySQL database in RDS and have tried all sorts of things in changing the security group to ensure I can get passed the Firewall (here's the tutorial I've been following https://medium.com/@ryanzhou7/connecting-a-mysql-workbench-to-amazon-web-services-relational-database-service-36ae1f23d424). I've been trying to create a new connection on MySQL Workbench. The hostname, port, user, and password are all correct so I imagine this is an issue related to the security group. When I run the command line client in MySQL Workbench I get the message, "ERROR 2003: (HY000): Can't connect to MySQL server on 'endpoint' (10060).

Any idea how to troubleshoot this? Again, I've edited the inbound rules on the security for both a universal IP and my computer's IP. If it matters I set it to public but I can't ping the endpoint when I copy that in my browser as a URL. Thanks in advance!