To enable ADB debugging on a Xiaomi device please follow the steps below. You will need a SIM card and a Mi account.
To enable Developer Options you need to locate the MIUI version in the device settings.
Settings > About phone > MIUI version
Tap 7 times on the MIUI version to enable Developer Options
Once enabled you then need to head back to the main settings page and navigate to developer options
Back out of About phone > Settings
Settings > Additional Settings > Developer Options
Please note that in order to enable USB Debugging mode on some Xiaomi devices you will need a Mi account. You can do that here: https://global.account.xiaomi.com/pass/register
To enable Install via USB and USB Debugging (security settings) please follow the steps below;
Insert a SIM card into the device
Additional Settings > Developer Options
Enable Install via USB > This will require you to sign in to your MI account
Enable USB Debugging (Security Settings) > Click through the 3 warning prompts
Now you can remove the SIM card
On some devices, you will now be able to log out of the Mi Account in the settings menu.
On some devices a prompt may appear to install the app, please click install.