|"One is connected to a cable box which is connected to the router by the cable/coax line"|
Nope doesn't work like that.
Since the coax devices are not connected to the lan side of the router they will not and I repeat not show up in the routers attached device table.
That would be like saying your neighbors mail is delivered to your house because you are all on the same street.
Make sure your wireless on the router is secured. Doesn't sound like it is.