aws-lambda-dotnet: Can not connect to MongoDB Atlas

I wrote a simple AWS Serverless ASP Net Core 2.0 WebAPI This connects to a MongoDB Atlas Database. But it is not working. It is giving timeout. I test locally and it Works. I publish to Azure it Works.

I suspect this is being caused by AWS CloudFormation firewall .

When I publish to AWS Serverless , it gives error:

502 Bad Gateway

{ “message”: “Internal server error” }

I checked the logs,

Unhandled Exception: System.TimeoutException: A timeout occured after 30000ms selecting a server using CompositeServerSelector{ Selectors = MongoDB.Driver.MongoClient+AreSessionsSupportedServerSelector, LatencyLimitingServerSelector{ AllowedLatencyRange = 00:00:00.0150000 } }. Client view of cluster state is { ClusterId : "1", ConnectionMode : "ReplicaSet", Type : "ReplicaSet", State : "Disconnected", Servers : [{ ServerId: "{ ClusterId : 1, EndPoint : "Unspecified/cluster0-shard-00-00-3lmjx.mongodb.net:27017" }", EndPoint: "Unspecified/cluster0-shard-00-00-3lmjq.mongodb.net:27017", State: "Disconnected", Type: "Unknown" }, { ServerId: "{ ClusterId : 1, EndPoint : "Unspecified/cluster0-shard-00-01-3lmjx.mongodb.net:27017" }", EndPoint: "Unspecified/cluster0-shard-00-01-3lmjx.mongodb.net:27017", State: "Disconnected", Type: "Unknown" }, { ServerId: "{ ClusterId : 1, EndPoint : "Unspecified/cluster0-shard-00-02-3lmjq.mongodb.net:27017" }", EndPoint: "Unspecified/cluster0-shard-00-02-3lmjq.mongodb.net:27017", State: "Disconnected", Type: "Unknown" }] }.

How to debug and solve this?

My serveless.template

{
  "AWSTemplateFormatVersion" : "2010-09-09",
  "Transform" : "AWS::Serverless-2016-10-31",
  "Description" : "An AWS Serverless Application that uses the ASP.NET Core framework running in Amazon Lambda.",

  "Parameters" : {
    "ShouldCreateBucket" : {
      "Type" : "String",        
      "AllowedValues" : ["true", "false"],
      "Description" : "If true then the S3 bucket that will be proxied will be created with the CloudFormation stack."
    },  
    "BucketName" : {
        "Type" : "String",
        "Description" : "Name of S3 bucket that will be proxied. If left blank a new table will be created.",
        "MinLength" : "0"
    }
  },

  "Conditions" : {
    "CreateS3Bucket" : {"Fn::Equals" : [{"Ref" : "ShouldCreateBucket"}, "true"]},
    "BucketNameGenerated" : {"Fn::Equals" : [{"Ref" : "BucketName"}, ""]}
  },

  "Resources" : {

    "ProxyFunction" : {
      "Type" : "AWS::Serverless::Function",
      "Properties": {
        "Handler": "AWSServerless1::AWSServerless1.LambdaEntryPoint::FunctionHandlerAsync",
        "Runtime": "dotnetcore2.0",
        "CodeUri": "",
        "MemorySize": 256,
        "Timeout": 30,
        "Role": null,
        "Policies": [ "AWSLambdaFullAccess" ],
        "Environment" : {
          "Variables" : {
            "AppS3Bucket" : { "Fn::If" : ["CreateS3Bucket", {"Ref":"Bucket"}, { "Ref" : "BucketName" } ] }
          }
        },
        "Events": {
          "PutResource": {
            "Type": "Api",
            "Properties": {
              "Path": "/{proxy+}",
              "Method": "ANY"
            }
          }
        }
      }
    },

    "Bucket" : {
        "Type" : "AWS::S3::Bucket",
        "Condition" : "CreateS3Bucket",
        "Properties" : {
            "BucketName" : { "Fn::If" : ["BucketNameGenerated", {"Ref" : "AWS::NoValue" }, { "Ref" : "BucketName" } ] }
        }
    }
  },

  "Outputs" : {
    "S3ProxyBucket" : {
        "Value" : { "Fn::If" : ["CreateS3Bucket", {"Ref":"Bucket"}, { "Ref" : "BucketName" } ] }
    }
  }
}