How to hard reset MI tablet when the volume down key and power key combination leads to fastboot screen
You can wipe the userdata partition  in Fastboot mode. 

Connect the device to an USB cable. Install drivers if needed, you'll have to download them. 

Download Minimal ADB and Fastboot (link 2), it a command-line utility. Open it and tape, the following: 

fastboot -w (confirm by hitting ENTER)

The device shall reboot and all settings reset.