I have an account with two unlocked RN4X. Got a new RN7 a week ago. I had to login to my Mi Account on the phone, so that the phone gets linked with the account, then in the developer options enabe OEM unlocking and the other unlock slider (don't know the name, sry). Then I had to wait 7 days until the MiUnlock tool allowed the unlock (check after setting up everything, so it will tell you how long you have to wait).