How can I install "ethernet" for Lenovo T430s?

All I do with that machine is router configurations

I suggest that you connect the ethernet cable to a LAN port on the router. Find the address of the router from the user guide or Google fu. For example, the router I configured yesterday was 192.168.8.1.

On the Ubuntu machine, from the terminal:

ip addr show

Find the interface name for the ethernet; for example enp0s25.

Back to the terminal:

sudo ifconfig enp0s25 192.168.8.2 up

Open a browser and navigate to http://192.168.8.1

The administration page of the router should appear.

Of course, substitute your details for those above.