Xiaomi
Volume Key Shortcut for Accessibility Features auf Xiaomi
Holding both volume buttons for three seconds is meant to trigger an accessibility function you picked, such as TalkBack or color correction. If nothing happens, MIUI has either turned the shortcut off completely or restricted it so it only works after unlocking the phone.
To check and adjust it, open Settings ➔ Additional settings ➔ Accessibility ➔ Volume key shortcut. From there, use Enable or disable Accessibility to switch the whole shortcut on or off.
If the shortcut is on but still not launching the right function, tap Shortcut service and use Choose a Shortcut service to confirm which feature is linked to the button combo. Some services need an extra confirmation via Allow before MIUI lets them run in the background.
After going Back, check Enable or disable Allow from lock screen. With this off, the shortcut only reacts once the phone is unlocked, which explains why it can seem broken when you try it from a locked screen.
This setting is worth revisiting after resetting accessibility options or switching to a different shortcut service, since MIUI does not always carry the lock screen permission over automatically.