@Tanmoy 

Do you mean only the upload speed is slow when you connect to the C6 router, is the download speed normal? May I know the below details then we will try to help you out?

1. Who is your ISP and what is the model number of your modem? What kind of connection type on the Archer C6, Dynamic, or PPPoE?

2. What is the download/upload speed you can get when you connect a computer directly to the ISP modem?

3. Do you enable any QoS setting on the Archer C6? Kindly disable it and show me the QoS setting page on your router.

4. Try to do a MAC clone on the C6 router:

https://www.tp-link.com/support/faq/1556/ (Case 1)

5. If none of the above helps, try to change the DNS server on the C6 Internet/WAN page to 8.8.8.8 or 1.1.1.1, then reboot the C6 to confirm again.

@Tanmoy 

I see. Are there any advanced or special settings on the C6 router for the LAN devices? If possible, reset the C6 to factory defaults by holding the reset button on the back panel until all LEDs flash off then on, reconfigure it from scratch, then confirm again.

The tech team was made aware of your issue. They will likely reach out to you via email to continue to troubleshoot your issue.