Now that the pressure is off and you've managed to download your Kindle books, I found another link with video clip that 'might' assist you to set up the Kindle with that USB cable you tried.
Concerning the THREE cables coming out of the Homebox. You just need to follow the cables and see which one (if any of them) go into your Desktop computer.
One cable will be from your outside internet antenna.
One 'obviously' will be the power supply from the homebox to the electricity socket
The other one is at my best guess, the cable (Ethernet) connection from the Homebox to your computer.
If my guesses and assumptions are correct then, I think that this cable would override your WiFi and why the Kindle can't find a signal.
The easiest options are - continue to use the local bar or, get someone who knows how to set-up the WiFi for you. It might be that you would have to unplug a cable to get the WiFi and then plug the cable back in again when you want your computer internet connection.
Best I can do working virtually blind, but I am confident that you will eventually sort it all out.