Can’t connect to RDS instance from EC2 instance

From EC2 instance i-78a8df00, I’m trying to connect to RDS instance mysql.************ They are both in the U.S. East region. I added the security group of EC2 instance (sg-********) to the RDS security group, but that didn’t help. It appears to be a firewall/DNS issue as it is timing out when running this command: