Timeout error for AWS SNSClient Publish request

Here is the piece of code :

                        //Publishing the topic
                        snsClient.Publish(new PublishRequest
                        {
                            Subject = Constants.SNSTopicMessage,
                            Message = snsMessageObj.ToString(),
                            TopicArn = Settings.TopicArn
                        });

I am getting the below error :

The underlying connection was closed: A connection that was expected to be kept alive was closed by the server.

And here is the screenshot of detailed error:

But not able to get an idea how to solve this. Any hint or link will helpful.