Some devices (eg: HuaWei, XiaoMi, Oppo) have built-in optimization features like 'Battery Saver', 'Data Saver', and 'Task Manager' which automatically closes/shuts down apps. This may be a reason why your ookyo VPN keeps dropping.
You'll need to go into your phone settings to stop it from closing the ookyo app. Please follow the steps here https://plus.google.com/109774443246850020262/posts/JHFE887Xe1u
Tip: You need to have a valid Unlimited App or Pop Deal to get unlimited internet usage on that app. It only applies to activities within the app.
Example: with an Unlimited App for Facebook, all usage within Facebook is unlimited. Like browsing timeline & profiles, watching videos, and viewing pictures.