Can you help me with this

Say you had a command line setup with nc on a windows box behind a router
and you want to use nc to connect to it somthing like this

router wan xxx.xxx.45.54 and lan
and a Box at with nc listening on 8080

i know that i could forward 8080 to but what if you can't access
the router and know the wan ip but not the box ip how could you get a connection to nc in this situation, simply connecting to xxx.xxx.45.54 8080
will not do as its not setup to forward 8080 to the box.