Fastboot should normally be available all the time. As already mentioned, UT is not locking anything so you should really be able to boot to fastboot and it is very weird that you can not.
Other than that, the first Google result for stock rom gave me this:
https://xiaomifirmwareupdater.com/miui/lavender/
which is the site I normally install stock roms for Xiaomi devices (works on Linux too). But basically (and while I know little about it) I believe you need to have either adb or fastboot working.
It is really hard to help you not knowing more information. I don't know how deep we should go.
E.g. How are you trying to access fastboot? Doesn't the usual key combination (probably volume down + power button) work?