Good summary of the facts. The only question now is, what is causing Linux not to see network shares or the USB drive on the router? I’ve done everything I’ve asked to do, and nothing works.
Try logging into the router user interface and check whether the router sees the linux machine. If you can’t log into the user interface from the linux box that is a useful piece of info, and of course if linux can’t log in you should log into the user interface from a windows machine and check what the router shows for the linux machine.