Both Ubuntu and Windows recognize my mi10 phone on normal startup, but when I've enter fastboot (Power+Vol Down) to burn a new rom, it is undetected by either adb\fastboot, I've enabled usb-debugging and OEM Unlocking of course.
After wasting good couple of hours,
It turns out you need to go the mobile device "Developer options" --> "Mi Unlock status"
Grant permission to "Mi Unlock" a utility by xiaomi that can be downloaded from here (works on Windows).
this app will unlock your xiaomi device, making it possible to burn a new rom.
For me I had to wait 168 hours (1 week) before the app unlocked my phone (An excessive security measure?)