Server to AWS RDS - No connection could be made because the target machine actively refused it

I'm moving a website from a dedicated server to AWS.

First I want to transfer the database (sql server express).

I've setup an AWS RDS Sql Express database - and can connect to it from my PC. However when I update the database connection string on my current server - to point to the AWS RDS Sql Express, I get the error:

[Win32Exception (0x80004005): No connection could be made because the target machine actively refused it]

[SqlException (0x80131904): A network-related or instance-specific error occurred while establishing a connection to SQL Server. The server was not found or was not accessible. Verify that the instance name is correct and that SQL Server is configured to allow remote connections. (provider: TCP Provider, error: 0 - No connection could be made because the target machine actively refused it.)]

The Security Group the RDS belongs to has the following ports open:

One IP, is my local PC - the other is my dedicated server IP.

Any idea why my local PC will connect to the AWS RDS Sql Express instance, but my dedicated servers connection is refused?

Is there any way I can find what IP address my server is trying to connect to AWS with (if it's not the public facing one)?

Thanks for any advice.

Mark