Unfortunately this Xiaomi phone has a very annoying procedure to unlock the bootloader: it is mandatory to execute an on-line procedure using a PC running an MS-Windows program, on the phone you have to enable the mobile internet connection (thus requiring a valid SIM card) and execute an app which will collect several data from the phone (IMEI, etc.). Once you start the procedure, you will be granted the right to unlock the bootloader, but with a wait time of several days. In my case the delay was about 14 days.