Measure Data Downloaded on TP-Link TL-WR841N Router: Monitor Internet Usage Across Devices

After all, this information is displayed on the screen. In the "Traffic Statistics" section you have information on how many bytes were sent and downloaded from the external network.

If you mean specific values for each user / device then this router does not keep such statistics. You can get them only by comparing the values from devices that you already know about using the network, for example the mentioned Windows 10. For example, by subtracting downloaded data from Windows 10 from the global value in the router, you get approximately the total value of downloaded data by other network users

As above, unless you use any alternative software for this TP-link, eg Gargoyle in which these statistics are more extensive.

In Gargoyle, there is not only counting for each device separately, but you can set the QoS after downloading / sending the amount of data.
In my opinion, this is the first thing that should be done with TP-Link devices - playing Gargoyle software.