A new application runs on Amazon EC2 instances and accesses data in an Amazon RDS...

The connectivity issues could be due to:

Security Group Ingress Rules:

Ensure that the security group for the RDS database has the correct ingress rules allowing traffic from the web server’s security group or IP address.

Go to the RDS console, select your database instance, and check the security groups.

In the security group settings, verify that the correct port (usually 3306 for MySQL) is open for the web server's IP or security group.

Port Configuration:

Verify that the application is configured to use the correct port that matches the RDS database configuration.

Check the application configuration files to ensure the port number matches the port specified in the RDS instance.

Amazon RDS Security Groups

Troubleshooting RDS Connectivity