Can be done using my firmware. First, I would strongly recommend you use SSH instead of telnet. Telnet is completely unencrypted, and a security risk when using over the Internet.
Create a firewall-start script (see README for details) with the following entry:
If you are running the original Asuswrt, then you will have to manually run this command over telnet every time you reboot your router or restart the firewall on it. Also, the original firmware does not support SSH.