The best choice without a cable is a pair of high quality powerline adapters like the Zyxel PLA5205kit (or 5215kit that has passthrough plugs). You can either directly connect to the computer or attach a cheap router configured as an AP if you want wireless up there for other stuff, and the other LAN ports will work like a typical wired switch that you can attach to the computer if you want.
You get what you pay for with powerline adapters, the cheap ones don't work well, but Newegg had the PLA5215kit on a good Black Friday sale that may still be available.