Help to debug AWS RDS export to S3

Using aws extension to export data from AWS RDS to S3 are described in
* https://blog.skyvia.com/postgres-to-amazon-s3-export/
* https://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/postgresql-s3-export.html#postgresql-s3-export-overview
I received error
ERROR: could not upload to Amazon S3
DETAIL: Amazon S3 client returned 'Unable to connect to endpoint'.
CONTEXT: SQL function "query_export_to_s3" statement 1


I would need help to troubleshoot the problem and make a basic export from RDS Postgres to S3 work.
I know this to be due to network configuration and connectivity. Therefore I would expect help from a AWS networking expert with some dev experience.