Can't get to fastboot. nor twrp nor OS - Xiaomi mi 5X/A1
Have you tried using adb? 

If you haven't got it already, download the SDK tools for windows from https://developer.android.com/studio/ under the "Command line tools only" section, and unzip it where you want.

Once you've done that, open a command prompt in the platform-tools folder within the SDK directory. The command

adp devices


will tell you that your device is connected. Then you can try

adp reboot recovery


If that doesn't work, install the universal adp driver and try again.