I think Mendix is mostly toying around with it in their labs and pretty confident that they will be able to set this live sometime soon. There is already some documentation on it: https://docs.mendix.com/refguide/ar-widgets. But no trace in the widgets indeed. Neither in the Appstore.
O, wait. How about: https://docs.mendix.com/howto/mobile/how-to-ar-simple-cube