fastboot devices recognizes the device and I can also boot the phone to fastboot: it shows a black (not blank) screen with the word FASTBOOT printed orange.
I've never used miui devices but I think you need to make an account and use this https://en.miui.com/unlock/download_en.html so you will probably need to have wine installed to run it, unless it has a native linux version in the zip file. Then you should have an unlocked bootloader