I’ve run cables between floors, down stairs and such. Stairs make things easier, because you can run the cable across them without tripping anyone, or needing to cover them. You can wind through bannisters to avoid using tape too.
Also, you should only be running one cable, and putting a switch at the other end.
I think you need to find out what kind of connection you actually have. I mean, it’s unlikely you’ve got a dedicated ethernet cable to the ISP.
Where does that ethernet line actually go?
Have you run some basic traceroutes, at least?