Why am I unable to connect to my AWS SQL server db in my MVC application?

I have a small MVC project that I want to connect to an AWS RDS SQL Server instance.

When I try to connect to it (via a test) I get:

Cannot open database "database" requested by the login. The login failed.
Login failed for user 'user'.

I have the connection string defined in Web.config like this:

<connectionStrings>
  <add 
    name="SystemDbContext"
    providerName="System.Data.SqlClient"
    connectionString="Data Source=someInstance.rds.amazonaws.com,1433;
    Initial Catalog=babychange;User ID=user;Password=somePassword"
  />
</connectionStrings> 

I have checked the DataSource, User ID and Password several times and am sure they are correct.

I am able to connect to the instance (via a Data Link UDL file) and really don't know what to do next

Can anyone help please?