@atanu
Sorry to hear the speed is not as good as expected, please help verify some details and we will try to fix it together:
1. What is the internet speed you can get when you connect the computer to your onu directly? With Archer C6 connected to the modem, please connect the same computer only to the Archer C6 and verify the speed again.
2. When devices connected to the Archer C6, are both wired speed and wireless speed slow or only 2.4G is slow? Please also verify the wired/wireless negotiation speed of your computer/smartphone so that we can rule out whether it is the negotiation speed which affects the actual speed.
https://www.tp-link.com/support/faq/2265/
3. Ensure that the firmware is up to date.
4. And try to disable NAT boost under advanced-system tools-system parameters page.
Any updates, please let us know.