There's two ways to telnet into a PC behind a router.
The first way assumes the \"router\" is a computer. Telnet into the router, then from there telnet into the PC behind the router. The router is connected to the internet so you can telnet into it from anywhere, and the PC behind the router is seen by the router...hence how it works.
The second way would require you to setup the router to forward a certain port to the PC behind it. For example, you could have telnet listen on port 77, then have the router forward all packets to port 77 to the other PC. Just make sure you use a port that no other PC on the network will need, as that port will ALWAYS be forwarded to that one PC.
