Using REST API and Android studio to retrieve data

Hi Everybody
Sorry for any inconveniences, both in terms of English and knowledge ;-).

Im currently creating a system, where a device will send its deviceID and geographical postion (latitude and longitude) in a Json format to TTN. TTN then forwards the payload to a data storage (using the data storage integration). An app should then fetch the payload from the server, which I could learn from the internet should use a REST API.
The payload received on TTN is correct, the same goes when TTN forwards the payload to the server.
The problem arises when I have to fetch the data from the server. I searched on the internet(and TTN), and tried some simple methods, but none of them gives me the actual payload.

Are there any of you guys that have a solution or can get me started in some way. I will really appreciate any help given!

regards