connect to mongodb on separate ec2 instance

I am running two different instances on AWS, one for node application and other for mongoDB. I am trying to connect to mongoDB on other instance but not able to and failing with "504 Gateway timed out".

I have commented the "bind_ip" in mongodb.conf and restarted mongoDB.

Also, I have opened the port 27017 for the node application server's public IP from the security groups of mongoDB instance for both inbound and outbound but of no use.

Please suggest a way to achieve this(if there is any). Thanks in advance :)