It is not hard from a unfinished basement to the first floor. More difficult to the 2nd. I have some leftover cat5e cables and can have a look if you like.
It should have been : use of air return duct. The air return duct on the second floor will go direct (straight down) to the basement, it's very easy to fish a wire down. Best way is to tie a wire, string with some heavy weight. By the way, most the air return duct on the second floor will be located close to the ceiling. you just need to make an opening near the base board (straight below the air return vent). Cover it with the RJ45 cable outlet cover.
Or use the same method to run CAT5E cable from the modem, cable or dsl to the main floor via the return duct if you cannot fish it through with an efficient way. Connect the WiFi Router at the main floor and you shuold be able to have good signal everywhere in your house.