Fastboot won't recognize the phone unless it's in fastboot mode. I'm not sure about the drivers. How did the install go? Do you ever see TWRP on your phone screen?
Installed latest stable build on my MI 11 Ultra from China ROM. Tried to skip the wipe --> format step after installing the unblocked zip file but it needs to be done. Otherwise my phone would reboot to TWRP. When I went to reboot prior to formating - TWRP would report No OS found! Are your sure...