Have you considered the AWS Transfer Family to transfer the files to a target such as S3? Then enable the ECS task to process the file from S3 as a source. It might enable you to leverage the service which interfaces with STFP servers natively and then enable ECS to process the file.
As Henrique suggested, I'd first spin a client (EC2 most likely) you can troubleshoot whether you have actual network connection possible from your VPC to the remote SFTP server. If that does not work, nothing will. If you are getting connection refused/timeout, there is either a firewall rejecting/dropping your packets or no route at all.
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