I was able to work with MindSphere without any issues.
Maybe following resources can help you to figure out the problem. Here is one about getting the data
Here is a sample application on the Marketplace which I also used as a foundation to build my own app
As a side note, please check about the security and access settings. Are you including the MindSphere access token in HTTP headers of the request from postman? I did not use postman, but in Mendix I had to provide some access token to be able to get data
Having spoke to a developer (and having a poke around myself) Mendix-based applications cannot be opened up to external calls. They can consume endpoints within MindSphere, but cannot allow themselves to be consumed by something such as postman (or external applications).