sorry for the very late response (it's better late than never, isn't it?), but Xiaomi has its bootloader unlocking service active. I used it last year to unlock my phone.

Here you can get the official mi unlock program (it works on hyperos): https://en.miui.com/unlock/index.html

And here is a guide to unlock the bootloader: https://miuirom.org/updates/xiaomi-bootloader

(don't do the usb debugging part, I don't know if it is necessary on older devices but on every xiaomi device that I had you don't need to enable usb debugging so mi unlock can communicate with fastboot)

edit: spacing.

edit 2: please don't do the bootloader lock via direct fastboot command as stated in the guide, I strongly recommend that you do a clean flash of stock rom with mi flash tool, if you plan to lock the bootloader later.