@DanQo 

You need to understand what QoS is:

"The QoS (Quality of Service) feature allows you to prioritize the internet traffic of specific devices to guarantee a faster connection when you need it the most. The router has preset an appropriate bandwidth allocation policy, so you don’t need to set different priorities for all devices. Simply set the desired device to High Priority and set how long you want it to remain the high priority, then the device will be allocated with more bandwidth when the network is overloaded. "

Login the router via its web-management interface at http://tplinkwifi.net or http://192.168.0.1

When TL-WR820N is in WISP mode I don't believe the QoS fetaure is available:

When TL-WR820N is in Router mode QoS is located between Parenatl Controls and Security:

  @DanQo 

I guessed that you want to limit the bandwidth of a particular device which is a different feature - Bandwidth Control.

That's why I tried to explain what QoS is and how it works. TL-WR820N doesn't have a Bandwidth Control implemented.