Please disregard my previous post. I saw the issue. It's on my driver settings. Need to switch Speed & Duplex from 100Mbps to 1.0 Gbps Full Duplex. Now it's working fine.

@Symptomatic I am encountering the same for my Archer C80. I am already using cat5e but LAN is just showing 100Mbps on the TP-Link's Ethernet status. My NIC is gigabit.

@PlastiBots Good to hear that the workaround works for you too. Hope TP Link to provide a new firmware to fix this issue.

Correction ****So I believe the issue is when both channels ends up either 20 MHz or 40 MHz. Another possible setup is to put 5 GHz to 80 MHz then leave 2.4 GHz to Auto.

I bought an Archer C80 just today and experienced the issue exactly as how most of you described it. I really can't understand why TP Link wasn't able to understand the issue fully considering that...