Xiaomi devices are a bit annoying to unlock but you should use the official way. I’ve got a xiaomi device and unlocked it. So first you need to be patient maybe it’s not going to work for the first time so try to unlock it and if it doesn’t work just wait a couple of weeks… And you should maybe ask a friend or family member to take their Sim card only to log into a xiaomi account and allow OEM unlocking. You could try an other method but when there is an official way (annoying but official) you should use it, safest and “easiest” way.