You don’t give many details on your overall setup.
Even so, I assume that you’re connecting to the Internet via a broadband router with NAT.
If this is correct, you need to configure “port forwarding” in the router that connects to Internet.
Basically, you need to tell you’re Internet access router that the telnet requests (port 23) you are willing to let get into you’re network, should be forwarded to the machine you’re trying to reach from the outside.
This is a simple process but router specific.
If you know what you’re doing, you can get some help here about port forwarding.
Once more, you must know what you are doing and configure it in a safe manner. Giving telnet access from the outside of this network will put it in risk. You should consider changing from telnet to ssh which is more secure.
Please post back if you have any further questions.