Timeout while connecting to DocumentDB from Lambda

I can successfully connect to the cluster (1 instance for now) from the Cloud9 console using mongo shell, however wasted hours trying to connect to it from within a lambda function.

Setup:

• Both cluster and lambda are in the same VPC (default)
• TLS is ON
• Cluster is in a security group called DemoDocDB which has inbound rules for 27017 for two security groups: cloud9 and DefaultSG
• Lambda is in the default VPC and also in the DefaultSG security group

Code:

• config.js

module.exports = {
    CONNECTION_STRING: 'mongodb://<user>:<pwd>@xxx.us-east-1.docdb.amazonaws.com:27017',
    SSL_CERTIFICATE: returnCerts(), // SSL Cert
    DB_NAME: 'documentdb', // Database name
    COLLECTION_NAME: 'events' // Tablename;
}

function returnCerts() {
    // Trick to avoid filesystem read of https://s3.amazonaws.com/rds-downloads/rds-combined-ca-bundle.pem
    return `-----BEGIN CERTIFICATE-----bla blah blah`
}

• index.js

const {CONNECTION_STRING, SSL_CERTIFICATE, DB_NAME, COLLECTION_NAME} = require('./config');
const MongoClient = require('mongodb').MongoClient;

let client = null;

exports.handler = (event, context, callback) => {
    
    client = MongoClient.connect(CONNECTION_STRING, 
    { 
      sslValidate: true,
      sslCA:SSL_CERTIFICATE,
      useNewUrlParser: true
    },
    function(err, client) {
        console.log('connection callback invoked')
        
        if(err){
            console.log(err)
        }    })              
    //callback();
    return {
        statusCode: 200,
        body: JSON.stringify({"message":"hey"})
    };
};

• Other: Nodejs 12.x, mongodb 3.6.2
• Error:

START RequestId: 5e135853-063b-4d5a-8a21-9a29d15c8750 Version: $LATEST
2020-11-01T02:21:43.912Z    5e135853-063b-4d5a-8a21-9a29d15c8750    ERROR   (node:9) DeprecationWarning: current Server Discovery and Monitoring engine is deprecated, and will be removed in a future version. To use the new Server Discover and Monitoring engine, pass option { useUnifiedTopology: true } to the MongoClient constructor.
2020-11-01T02:21:54.053Z    5e135853-063b-4d5a-8a21-9a29d15c8750    INFO    connection callback invoked
2020-11-01T02:21:54.091Z    5e135853-063b-4d5a-8a21-9a29d15c8750    INFO    MongoNetworkError: failed to connect to server [docdb-2020-10-31-23-57-52.cluster-cgzg3t2i3zpn.us-east-1.docdb.amazonaws.com:27017] on first connect [MongoNetworkTimeoutError: connection 0 to docdb-2020-10-31-23-57-52.cluster-cgzg3t2i3zpn.us-east-1.docdb.amazonaws.com:27017 timed out
    at Socket.<anonymous> (/var/task/LambdaDBTest/node_modules/mongodb/lib/core/connection/connection.js:421:7)
    at Object.onceWrapper (events.js:421:28)
    at Socket.emit (events.js:315:20)
    at Socket._onTimeout (net.js:482:8)
    at listOnTimeout (internal/timers.js:549:17)
    at processTimers (internal/timers.js:492:7) {
  [Symbol(beforeHandshake)]: true
}]
    at Pool.<anonymous> (/var/task/LambdaDBTest/node_modules/mongodb/lib/core/topologies/server.js:438:11)
    at Pool.emit (events.js:315:20)
    at /var/task/LambdaDBTest/node_modules/mongodb/lib/core/connection/pool.js:562:14
    at /var/task/LambdaDBTest/node_modules/mongodb/lib/core/connection/pool.js:995:11
    at callback (/var/task/LambdaDBTest/node_modules/mongodb/lib/core/connection/connect.js:75:5)
    at /var/task/LambdaDBTest/node_modules/mongodb/lib/core/connection/connect.js:101:9
    at _callback (/var/task/LambdaDBTest/node_modules/mongodb/lib/core/connection/connection.js:329:7)
    at Connection.errorHandler (/var/task/LambdaDBTest/node_modules/mongodb/lib/core/connection/connection.js:344:7)
    at Object.onceWrapper (events.js:422:26)
    at Connection.emit (events.js:315:20)