Invalid permission from Lambda to MongoDB in EC2 - mongodb

Question

I have created a Lambda Function which intends to connect to MongoDB running on EC2.
I have followed some tutorials and guaranteed that:
Lambda and EC2 run in the same VPC
Lambda has configured EC2's subnet
Lambda has its own security group my-lambda-sg
Lambda's security group is allowed in EC2's security group inbounds rules for MongoDB's port as a "Custom TCP Rule"
Lambda's role has assigned permission AWSLambdaVPCAccessExecutionRole
However, I am stil unable to connect from the lambda to the MongoDB in EC2. When I run a Test in the Lambda I get:
START RequestId: f0869292-5207-11e8-85d2-cba0eb99208c Version: $LATEST
2018-05-07T15:04:29.117Z 567951c4-5207-11e8-ba43-0314179d2bf5 { MongoNetworkError: failed to connect to server [IP:port] on first connect [MongoNetworkError: connection 0 to IP:port timed out]
at Pool.<anonymous> (/var/task/node_modules/mongodb-core/lib/topologies/server.js:503:11)
at emitOne (events.js:116:13)
at Pool.emit (events.js:211:7)
at Connection.<anonymous> (/var/task/node_modules/mongodb-core/lib/connection/pool.js:326:12)
at Object.onceWrapper (events.js:317:30)
at emitTwo (events.js:126:13)
at Connection.emit (events.js:214:7)
at Socket.<anonymous> (/var/task/node_modules/mongodb-core/lib/connection/connection.js:256:10)
at Object.onceWrapper (events.js:313:30)
at emitNone (events.js:106:13)
at Socket.emit (events.js:208:7)
at Socket._onTimeout (net.js:420:8)
at ontimeout (timers.js:482:11)
at tryOnTimeout (timers.js:317:5)
at Timer.listOnTimeout (timers.js:277:5)
name: 'MongoNetworkError',
message: 'failed to connect to server [IP:port] on first connect [MongoNetworkError: connection 0 to IP:port timed out]' }
END RequestId: f0869292-5207-11e8-85d2-cba0eb99208c
From API Gateway I get
You do not have permission to perform this action
And from the browser I get:
{"message": "Endpoint request timed out"}
Other Lambda's not accessing EC2 are working fine.
Any idea what could I be missing?

Answer 1

I found the answer. All requisites above are ok. I just needed to use the internal IP of the EC2 instead the external one in the MongoDB URL connection, to be considered a call within the VPC, as I found here AWS Lambda unable to access EC2 port within the same VPC