My router is also downstairs and that's what I did. I bought a 100ft cable that came with the cable clips and ran the cable along the bottom of the wall and up the stairs. I switch my cable between my pc and my switch depending on what I'm playing, it's great.

It sounds daunting but it really didn't take that long and I did a pretty good job so you can hardly notice the cable is even there unless you look for it. It helps that I bought a flat ethernet like this one. In fact, this might be the exact one I got.

PS pardon the Amazon link fuck bezos buy local