I have just upgraded my old ADSL internet connection to Gigabit fiber optic.
I have an Archer C80 which was previously plugged into the slow ADSL modem without issue.
When I plugged it into the new optical modem, the C80 will not connect and state that WAN port is disconnected. 
After some troubleshooting, I've found out that the C80 will connect if negotiation mode is set to 100 Mbps Full Duplex (or lower), but not if left set to Auto or set to 1Gbps.

I also have a brand new Archer AX12 which I purchased to replace the old ADSL modem regarding WiFi coverage on a different floor. 
I tried to swap these two routers, and when the AX12 is plugged into the new optical modem, over the exact same Cat5e cable, it works without issue. 
The LAN ports on the C80 work fine, again over that very same cable when the network branch is reversed (i.e. I swap the cable from WAN to LAN).

As a last resort, I have also tried to plug the C80's WAN directly to the AX12's LAN port through a short 1m Cat5e patch cord, with same results. The C80 will still not accept full speed on the WAN port. 

I have also tried to factory reset and firmware update the C80 with no change. 

I have no idea if this is a new fault or if it was always like this, as I never had a device capable of speeds above 100 Mbps plugged into the WAN port before now. 

Any help would be greatly appreciated.