Don´t know if i understand correctly - can you see the countdown timer for unlocking?
If not, maybe you can try to reset and use another sim card for unlocking process. Here are some tips from another user with unlocking problems and steps how he succeeded: https://c.mi.com/thread-2782946-2-1.html.
I unlocked my 10T Pro successfully with the same version (5.5.224.24) using an USB 2.0 Port. It is often mentioned that USB 3.x Ports are not recommended.