You tried the virtual server option in the web interface I guess?
you can try ip-tables, it just hard-codes it directly in to the firewall
the usual iptables script:vi /tmp/local/sbin/post-firewall
(if you have "iptables -D INPUT -j DROP" and "iptables -A INPUT -j DROP" don't add them again)
ofc, if you never enterd any code make it executable:#!/bin/sh
# deleting last firewal rules (policy)
iptables -D INPUT -j DROP
# Standard open ports
iptables -A INPUT -p tcp --dport 51413 -j ACCEPT #opens up tcp port 51413
# Restablishing INPUT chain policy
iptables -A INPUT -j DROP
and save + reboot:chmod +x /tmp/local/sbin/post-firewall
doneflashfs save && flashfs commit && flashfs enable && reboot