Unfortunately, as far as I know, this is not possible with this model (TL-MR6400).
However, the following device supports this: Tp-Link Archer MR600 (~CHF 130).
However, you have to log in to the router with a web browser and go through several submenus until you get to the traffic monitor. The mobile app from TP-Link ("Tether") does not yet support this. (I have tested the iOS version).
I myself use both TL-MR6400 models and the TL-MR600. The web interface looks exactly the same, with the TL-MR6400 some things have simply been removed, such as the traffic monitor and internet speed test.