The AWS Marketplace provides a "launch with 1-click" button to simplify deployment of Stingray in EC2.
If you accept the Marketplace defaults a new security group is created for you automatically. The security group is fine for single instances of Stingray, but attempts to cluster traffic managers associated with this security group will fail.
Prior to clustering traffic managers, modify your security group to open ports 9080 and 9090 for UDP traffic. For more information see the 'Before you begin' section in the Getting Started Guide for Stingray Traffic Manager on EC2 - see Stingray Product Documentation