Check your hostname and port once,
Please try port number 5433 once. As this is the default port number.
Hope this helps!!
It may be because of firewall setting at cloud end.
It is not allowing your application to connect with cloud instance.
Check the firewall settings.
When you check your postgress Connection settings (right mouseclick → properties) you will find the settings for your connection.
make sure it is set to 5432, like it is above.
In Mendix, just connect to the postgress server
in your case; replace ‘localhost:5432’ with corpmendixpsqld1.cluster-cbcnpoyntsu7.us-east-1.rds.amazonaws.com:5432
Hope this will get you going,