@ltpozzato 

Hi,

Please try the below suggestions and get back to the community with the details if it cannot be solved, thanks.

1. Are the C7 and C80 connected as the below?

ISP modem--(WAN) C7 (LAN)--(LAN) C80 (LAN)--PC, is the C80 working as an AP?

Or ISP modem--(WAN) C7 (LAN)--(WAN) C80 (LAN)--PC, is the C80 working as a router?

2. What if you connect the PC directly to the C7 or the C80, how about the negotiation speed on it, 100Mbps or 1000Mbps?

3. Log in to the web UI of the Archer C7 and go to the Basic-Network map page, click the Archer C7 image/icon and scroll down this page, you will see the Ethernet negotiation speed of the LAN ports, show us the screenshot:
https://www.tp-link.com/support/faq/2472/
You may go to Advanced-system tools-system parameters to edit the duplex, change it to 1000Mbps full-duplex to give it a go, then test the negotiation speed on your PC again. It is strongly suggested changing another cat5e or cat6 cable. 

4. It is also suggested changing another wired device with gigabit Ethernet ports to do a test. 

Thanks.