Hello @Clochix and @ZeHiro,
thanks you both for reply and sorry for my late answer.
I don't have a Mi Band 2 right now. I want to find a way to store my data directly on my own server without using Xiaomi's (or who ones ever). But if I understand you correct is, that this can not work. I need the API from the provider.
Maybe there is any other fitness tracker for realizing my real own tracker.