The Mi Box app allows you to control the Mi Box from your smartphone.
Attention, this isn't the official Mi Box app, but with this remote control app, you can control it.

We offer a variety of remotes, so you can choose the one that suits your device.

The app was designed to help you use your Mi Box device even if you can't find your remote control! However, you should note that the app requires your phone to have an IR sensor.