Linksys's Support personnel struck out on this, but here goes:

I have a Linksys wireless router, model no. WRT54GS, v.7.
I have a desktop, a laptop, and a printer connected to the router
via cat 5 cables running10Mb Ethernet. The router also connects
to a cable modem via cat 6 cable running 10Mb Ethernet. I want
to move the router, desktop, laptop, and printer upstairs in my condo.
The cable modem must remain downstairs where the coaxial cable
terminates because of difficulties in running a cable upstairs. (Please
just believe me.) The problem is how to link the cable modem to
the router by wirelessly simulating an Ethernet cable. Is there
a device (or devices) that can do this?