@Tasfiq
Hi, the Archer C6 v3.20 does not support bandwidth control (limit bandwidth per device), while it supports QoS, you can refer to the below instructions to get the configuration guidance:
https://www.tp-link.com/support/faq/1708/
We have feedback to the related department, and the bandwidth control feature will be available on the wireless routers in the future firmware update, but we cannot provide you an ETA, we cannot know which model will have it at the moment.
You are suggested to pay attention to the official website, or the new firmware notifications on your router web GUI for any new firmware updates.