Firstly, If you get No content response(error code:204) from calling the GET API, it might be due to the fact that you haven't sent Cloud-To-Device messages. It basically, means the server has fulfilled the request, but there is no new information to send back.
Secondly, Device-To-Cloud message you have sent is actually unreachable (as the GET API is meant for Cloud-to-Device Messages)
Finally, sending a Cloud-To-Device message.
Just for testing purposes, try sending Cloud-To-Device message from Device Explorer.
Hope it helps! Good Luck.