Hi all,

I have a 500/50 connection. Directly from my modem I get full speed without issues.
My Archer C80 shuts the WAN port down as soon as I set its negotiation speed to 1000 Mbps. Auto negotiaton goes to 10 Mbps full duplex. I can force it to 100 Mbps full duplex, but I have to reboot the router to make that one work. Even after the reboot I first have to disconnect and reconnect the ethernet cable.
The device is in router mode and I intend to keep it that way.

I've tried so far:
- power cycling the router and modem quite a few times (power off both, first start the router, then the modem when it's running, but I've tried it the other way around as well)
- 4 different CAT 5e cables and 2 different CAT 6a cables
- factory reset
- disabling NAT boost, enabling QoS, limiting the bandwith to 999/999 and various other values
- disabling NAT completely
- turning IPv6 on and off
- cleaning the WAN port with air and a soft brush

When I powered up the router it was on an older fw version, I didn't check which one, but I only updated later. So I guess fw issue can also be ruled out.

In the network map the WAN port sometimes lights up and shows 1000 Mbps Full Duplex for a few seconds, but then it turns grey again. During these short periods I still get no connection and the light on the output port of my modem doesn't light up either.

WiFi speeds are the same as cable speeds by the way.

I guess the router is faulty? I'm out of warranty as it's 3 years old. I have used it in the past the same modem at a different address, and it worked perfectly fine. But I wasn't using it in the last 2 years. 

Many thanks for your help in advance!