I bought an osmo mobile few weeks ago. And it's not stabilizing properly anymore. I tried to calibrate it again, still doesn't work. What should I do?  Bring it back to the hongkong store? Using iPhone 5s and 6s with it

Yes, you should send it back. DJI products only has warranty in the continent where it's bought. So if you bought it in Hong Kong, you need to send it to Hong Kong for an exchange.