After all, I have succeeded in ROOTing my Hisense Sero 5 (HS-L691), here are the instructions how you (hopefully, if successful, please write feedback/say thanks ) can (hopefully all) ROOT your Hisense Smartphones & Tablets.
You will need:
• Kingroot in the latest version (currently I have installed 4.6.0).
Official Homepage: www[dot]kingroot[dot]net
[If you want to try it instead with Towelroot, Framaroot or similar, please post here, if it was or wasn't successful. Thanks! ]
1) Start Kingroot and wait until your smartphone model and "Root access is not available" appears.
2) Go (via the notification bar) to Settings -> Device: Apps -> Manage applications -> All.
3) Look at the end of the Apps list for the App with Chinese characters and a lock as an App icon (see screenshot).
4) Click "Force stop" and confirm the warning message with OK.
5) Switch (on the most recently used apps, home button double tap) to Kingroot and press the "TRY TO ROOT" button.
6) Wait until Kingroot reports the success (and share it with others, if you want. )
7) Now you can install Busybox, the modified Play Store, Lucky Patcher etc. (or for what ever you need ROOT. ) and enjoy yourselves life and possibilities of your Hisense Smartphones and/or Tablet.