I used two different versions of Miunlock. I used version 4.5.813.51 in order to get an SMS sent. I also used a Google Voice number. This version might work with any number, but I was well and truly down the rabbit hole by this point. I then switched to version 6.5.406.31 for the unlocking. Because cookies are involved, I was not required to receive another SMS code.
Before attempting to unlock, I made sure that I had a working android bootloader device (version 11.0.0.0 from Google) in Windows device manager by installing it manually and pointing the "browse for files" dialog to the drivers folder of the miunlock version 6. And then I found when it stopped halfway in the unlock, that I needed to boot back to normal, log out of the Mi account, log back in to the Mi account and then go to the Developer options and the dialog for check unlock status. After getting that squared away, booted back into bootloader, connected back to the version 6 (which I left open where it left off) and clicked "try again" and it worked. Oh, I also went to i.mi.com and did the find my phone routine because somebody mentioned they did that to get it all working. This may or may not be necessary, I don't know.
Thank you Bobcov. Spent hours figuring this out, but your post was what worked for me.
Same situation here. Tried 3 local SIMs, 3 free SMS overseas service. And never got the SMS.
I always did get the SMS from updating the MI account from my computer's web browser, just not the SMS when trying the unlock tool.
What worked for me: 4.5.813.51 (but could've been -5.5.224.24 - sorry tried so many combinations...)
I entered my phone number (my local SIM#) & password (ticked Trust this device!), and the catcha/web page was slightly different from the newer versions. And the SMS did come through within seconds. Entered it. But for me, there was no unlock option....
Then I closed that version, and opened version 6.5.406.31 . I reentered phone number and pasword. But this time, no SMS was necessary. (cookies??) And I finally got to the unlock screen.
Didn't use VPN, nor Mi cloud. And QR code didn't work either for me.
Now I wait out the 168 hours.
Sheesh. what a hassle.