Attic/crawl space. If you don't have access to either there is a drill bit on a 40-80 inch flexible shaft. You cut a hole in the wall for the box you'll mount

Attic/crawl space. If you don't have access to either there is a drill bit on a 40-80 inch flexible shaft. You cut a hole in the wall for the box you'll mount the ethernet in and then use it as access to drill up/down/sideways behind the wall.