Maybe this command or maybe i am wrong but try this
In place "your port" you can use 27015,27016 etc.
1. iptables -A INPUT -p tcp --dport "yourport" -j ACCEPT
2.iptables -A OUTPUT -p tcp --dport "yourport" -j ACCEPT
3. /etc/init.d/iptables restart
OR this
https://m.wikihow.com/Open-Ports-in-...Firewall?amp=1 (not advertising )