what are you doing ? you dont need fastboot rom version if you have twrp
1. download the rom version 12.5.7 to your pc
2. start your phone not in recovery and not in fastboot, start it as always
3. copy 12.5.7 rom zip file from pc to your phone with usb
4. power off your phone and start it in fastboot (power and vol-)
5. open cmd and switch to the folder with fastboot.exe
6. the twrp.img should be in this folder as well
7. fastboot boot twrp.img
8. twrp should be displayed on your phone
9. change the language in twrp (the lower right button)
10. click on install in twrp
11. navigate to the folder where you have copied the 12.5.7 rom version
12. install it
In most cases you can copy the file from pc to the phone while you have started twrp
PS: on monday I can unlock the bootloader on my mi11 ultra and I will see if it will be a challenge