@archer_c6_user 

Hi,

Normally, if you set the Internet Port Negotiation Speed Setting to "1000Mbps Full Duplex" and click save, then the Ethernet port should either connect at 1000Mbps Full Duplex or, if the other side doesn't support 1000Mbps Full Duplex, not connect at all.

If you've fixed the speed setting to 1000Mbps Full Duplex, but it still connects at 100Mbps, then something weird is going on.

I suggest you perform a "Factory Default Restore" on your C6 and then try again.

In case that doesn't solve the issue, then you could try to downgrade your C6(US) to the same firmware version your friend is running on his C6(US).

https://static.tp-link.com/2020/202012/20201217/Archer%20C6(US)_V3.20_201117.zip

Thank you, @woozle. I tried both the solutions and the problem persists.

The Negotiation Speed for Internet after the "Factory Default Restore" is still 100Mbps Full Duplex.

Downgrading the firmware to 1.0.5 Build 20201117 rel.41066(5553) does not solve the issue. Setting the Internet Port Negotiation Speed Setting under System Parameters to 1000Mbps Full Duplex connects to the ISP but says that the Negotiation Speed for Internet is 100Mbps Full Duplex. My ISP has confirmed that the port on their side supports 1000Mbps(also proved by the fact that a direct connection to ISP from a Gigabit Ethernet port of a laptop gives 130-140Mbps).

Could there be a hardware failure in the router? Can this problem be diagnosed and fixed at a TP Link Service Center?

  @archer_c6_user 

If you have the same C6 V3.2 like your friend, with the same firmware now and after a fresh Factory Restore, then both routers should behave identical.

Since this is not the case, there could be a hardware failure. The only thing you can check yourself is the eight flexible contacts of the Ethernet port. (i.e. no contact broken off, all contacts are straight, no contamination on the contacts)

Is your C6 still under warranty?

  @woozle, the eight flexible contacts of the WAN port seem to be straight, clean and not broken.

It has been 13 months since the router was purchased. It is still under warranty.

  @Kevin_Z, I've tested the router with another ISP where the subscription speed is 150 Mpbs. My friend's router negotiates 1000Mbps Full Duplex for the same line whereas my router negotiates 100Mbps Full Duplex.

The Gigabit Ethernet port on my laptop connects upto 130-140Mbps with the ACT MODEM and my router does less than 100 Mbps.

The ACT MODEM is placed in an adjacent building(serves other subscribers in the neighborhood) and I can't try with another CAT5E or CAT6 cable from the ACT MODEM to my router. My ISP provider has inspected and confirmed that the ACT MODEM supports 1000Mbps Full Duplex(also proved by the Gigabit Ethernet port on my laptop).