@Kedarnath568 

Hello,

May I know who is your internet service provider and what is the current network topology? 

Without any devices connected, please connect one computer to the C6 directly and go to speedtest to run the test for 3 times and show us the screenshots of the test results.

And please tell me the wired link speed at that time, how to get that? You can refer to the below link.
https://www.tp-link.com/en/faq-2265.html

Meanwhile, please check the information mentioned above with your device connected to the modem directly so that we can do a comparative analysis.

What's the current hardware and firmware version? 

You can refer to the below instruction to narrow down the issue as well.

https://www.tp-link.com/en/faq-2170.html

Good day.