Well, your choices (as I see 'em) are:
1.) SSH to the router (Cisco only does SSH1 until version 12.3-something), then telnet / ssh from there.
2.) SSH through the router to a *nix box and Telnet / ssh from there. SSH packets look like any other and go through the router like spring grass through a goose (so to speak).
Going to the *nix box would be my (and nearly everyone else's I s'pose) recommendation. In addition to doing SSH2 (much more secure), it'll also give you the capability to do SFTP (secure FTP, basically FTP through an SSH tunnel).
Either way, you'll need to open a hole / port forward port 22 on the firewall (if the router is behind a firewall).
Good Luck
Scott