Appflow custom Connector jdbc connection issue

Hi, i have created an appflow custom connector for jdbc using https://github.com/aws-samples/amazon-appflow-custom-jdbc-connector I then want to create the jdbc connection to a public RDS i have also created. I can confirm that my RDS is accessible from Internet, but i have the following error while i try to create the connection

Error while communicating to connector: Failed to validate Connection while attempting "ValidateCredentials with CustomConnector" with connector failure The request failed because the service Source Customconnector returned the following error: Details: Client execution did not complete before the specified timeout configuration.. (Service: null; Status Code: 400; Error Code: Client; Request ID: null; Proxy: null)

i guess it is some connectivity issue but can't figure out where exactly it could be blocked.

Thanks