Have you access to root account ?
If so, try something like :
iptables -A INPUT -p tcp -i eth0 --dport 10000 -j ACCEPT
in this manner, you accept all tcp traffic to the port 10000 (webmin port)
Be carefull, at the next reboot, your new rule will vanish.
If you don't have access or don't want to, you can try through webmin/network/linux firewall/ add new rule.