Open a CMD prompt and type "ipconfig/all" to see what current ethernet adapter / local area connection(s) you have. If none, you'll need to go through setting up a new one. If you have a local area connection, you'll need to change some settings to get it to see the router/modem.
Find out the IP address and subnet of your router/modem (LAN IP; not WAN IP).
In PC network settings set your PC to have a similar static IP, same subnet mask, and enter the router/modem LAN IP as your default gateway.
e.g. if the router's LAN IP is 192.168.1.1 (subnet mask 255.255.255.0), you can set your PC to be 192.168.1.2, and subnet mask the same, and the default gateway is 192.168.1.1
DNS servers can be 1.1.1.1 (or other public DNS server) or ISP's DNS server IP numbers. Suggest you have a read of the router/modem manual or quick setup to ascertain the IP settings.