[Redmi K20 / Xiaomi Mi 9T Guides, News, & Discussio] Greenwaldist:
Recently tried to unlock the bootloader on a Xiaomi MI phone (9T). I didn't keep notes so I can't give an exact instruction, but here are some tips and tricks to use together with other more comprehensive step by step instructions. I had every problem imaginable, so these instructions are for worst case getting stuck situations.

The problems are this. The USB drivers on your computer must recognize the phone when you connect it with USB cable and the MI Unlock Tool must recognoze the phone. And Xiaomi will not allow you to unlock unless you have set up you MI account with Xiaomi in the correct way.

First, put in a new active SIM in the phone with a phone number that has never been connected to an MI account before. If the phone has been used with another SIM before with another MI account, do a factory reset to erase any trace of the previous account (don't know if this is necessary but I did it just to avoid any problems).

Create an MI account in the phone, and ensure that Cloud Sync and Find my Phone are activated (not doing this makes it impossible). Give MI cloud all permissions on your phone. (And enable USB debugging, etc. per all the other instructions.)

Reboot you Windows computer, and during startup click repeatedly on F8 to get into the BIOS set up (I used Windows 7 and I have seen comments that Win10 could be a problem). Once in the BIOS menu, select: Disable Driver Signature Enforcement. This is valid until next reboot and will make it easier to get the USB drivers for Xioami to work.

Download the MI Unlock tool. The offical website will give you the wrong app (batch_unlock.exe ). Do NOT download from: https en.miui DOT com/unlock/download_en.html

Once you have found the correct Zipfile for the MI Unlock tool, create a folder directly under C: with a short foldername and no spaces. Unzip in that folder. Start the tool by clicking on: miflash_unlock.exe. Log in with your MI account credentials. Then click on the cogwheel in the upper right corner of the MIflash app. Then check for the latest version. If a later version exists, put that zip in another folder directly under C: and install (as above).

Before clicking on miflash_unlock.exe, click on MiUsbDriver.exe in the same folder. This will install the USB drivers. Use a USB2 port instead of USB3 if possible.

If the MI unlock tool can't recognize the phone when you plug in the USB cable (phone must be in fastboot mode, etc.), the Unlock tool will say: Not Connected To The Phone . If so, check if there is a problem with the USB drivers ) or if MI Cloud Sync is disabled). Go to the Windows Device Manager, maybe uninstall older drivers, maybe install dowloaded USB drivers for Android/Xiaomi.

When I finaly could run the tool, it unlocked directly, without any waiting time. But if you get a waiting time, keep the SIM in the phone and have it on and use the phone during the waiting time to avoid that the waiting time gets even longer.

(And if you want to flash Lineage on to you device, don't install a larger package from Google Gapps than nano. Otherwise, the Pixel launcher that comes with Gapps will break Lineage.)