Possibly due to firewall. If not firewall then simply check whether there is any inbound ACL configured on your WAN interface or not. If there is one, try removing it and telnet from Internet. If works then you know that you'll have to edit your ACL to permit telnet. BTW, I do not recommend telnet on WAN/Internet interface. Since your IOS can do k9, check and see if you have SSH enabled or not. Once enabled, SSH to your router instead.
There's also a possibility of routing issue which might have default route pointing straight out of the WAN interface for your LAN network. Quite a lot os possibility here.
