you will have to flash custom firmware to get telnet access. telnet is automatic once you have flashed it.
On the other side, you will have to save your change to firewall using post-firewall script to get your settings after a power off.
If you just need to enter a quick command, you can try the hidden admin page (read the howto for this) but you will have to type your command precisely with limited feedback, hit refresh, and all change will be lost after reboot.