@ErickLennon2020 

So it's been a year and no resolution. And I have the same issue but I *want* the 100mbs negotiation, and get 10mbps..

@Marcin997 Same issue. Damn 10 mbps in internet port speed (WAN). My other router works fine with same internet, but this router archer c6 only negotiating 10 mbps.

I have this same problem. I changed the parameters to be 1000mbps, and everything worked fine for a while. Then suddenly the router restarts itself which cuts the whole connection for a couple of seconds(all green lights go out, and one orange light is flashing). After the router turns itself on again, the speed gets negotiated to 100mbps even though the parameters are still set to 1000mbps. In order to change it back to 1000mbps, I have to go in between auto negotiation and 1000mbps, which could take only a couple of tries, or so many tries that I give up, and just have the connection be 100mbps until I feel like trying again. The problem definitely is with the router, not ISP. I also tried to change the speed that my OS is trying to set to 1000mbps, but it made no difference. The archer c6 software still just negotiates itself to a 100mbps connection. The firmware also is up to date, so that isn't the problem either.

This to me sounds like an overheating- or overworking the processor kind of a problem, which is weird since the router is not in a secluded hot corner, and it should definitely be capable to handle speeds of 1000mbps. Also the software "fighting to change the speed back 1000mbps" should definitely not happen, and let me change the speed as I will. The router restarted itself even when I was just playing a game over the net, which definitely shouldn't be overstressing the router.

@ErickLennon2020 I had a similar issue. I have 1Gbps download speed from ISP and was getting close to that when direct connected to my cable modem, but when I connected to my ethernet LAN ports on the archer router, I was getting ~560Mbps.

I connected with support today, and they had me change the QOS settings to 1000Mbps. Now directly connected to the LAN ethernet ports I'm getting close to 1Gbps. 

Advanced --> QOS --> Application or Device Priority --> At the top it says Total Bandwidth

Click on the pencil edit icon and change upload and download from "Get via internet speed test" to set manually at 1000Mbps

Just sharing.. i have a 300 mbps service and my router (Archer C80)connected to the modem previously able to reach beyond 300mbps. The past few weeks(or months) i noticed a speedtest of only 90-95 mbps on the average. It actually does not affect me but thought of looking into it and got redirected to this site  thru google (thanks). So when I check my router's gateway page, i only see that the ethernet status (internet)  only has 100mbps duplex speed. I tried checking how to edit it manually until I saw the notification(red) that I can upgrade my router's software as it haven t been over the past few months. So i did, then its magic - the Ethernet status now updates to 1000 mbps. I did a speedtest check and my wifi now reaches 320mbps on the average.

@ErickLennon2020 just do a factory reset. It will solve the problem. Worked for me

Archer C6

Changed the ethernet cable with a new Cat5e and that seems to did the trick.

Also changed the "system parameters" settings under "System Tools" to 1000 Mbps Full Duplex.

Internet negotiation speed is now showing 1000 Mbps Full Duplex.

Can confirm the same by looking at the improved speed.