Try opening a terminal window by plugging in a keyboard to the controller then hold the Ctrl key, press T then C and let go of the Ctrl key.
Next type "ping 127.0.0.1” this called the loopback address and will test the ethernet port of the raspberry pi to make sure it is working, this should work with or without a cable plugged in. It should look like this.
It sends test data to the destination IP and lists the response. I think it says something like destination unreachable or something when it fails. Hold the Ctrl key and press C to stop the ping. If this works the ethernet port of the controller is good.
Next make sure the cable is plugged in and use the ping command again but this time use your router IP instead of the loopback. The router should respond the same way with no lost/dropped packets. If it can’t ping the router it’s probably a physical connection issue at the ports or in the wire. If it can ping the router it’s usually a software issue like a firewall or some setting in the router blocking the controllers access.
To get back to the controller display hold Ctrl, press T twice and let go of Ctrl.