@Jibril 

Hi,

I think this requires some troubleshooting, before anyone can make a conclusion.

First thing would be to replace the Ethernet cable with another known good one.

After a cable problem has been ruled out you would need another device that has an Ethernet port to rule out either the modem or the C80.

If you have a computer with Ethernet port, then connect it to the modem via Ethernet cable and check the behavior like you did before. (i.e. manually set different speed/duplex modes on the computer and check the result on the modem's status page)

After you've determined everything works as it should, then connect the computer via cable to the WAN port of the C80 and repeat the same procedure.

In this case you will have to check the actual link speed and duplex mode on the computer. In Windows this is a bit tricky, but the following command should work on Windows 8.1 and Windows 10. It needs to be executed in a "PowerShell" window (not the regular cmd prompt). 

Get-NetAdapter | Select-Object -Property Name, InterfaceDescription, MacAddress, FullDuplex, LinkSpeed