Install iptables-persistent and save the iptables to /etc/iptables
sudo apt install iptables-persistent
sudo iptables-save > /etc/iptables/rules.v4
sudo ip6tables-save > /etc/iptables/rules.v6
see also https://www.cyberciti.biz/faq/how-to-save-iptables-firewall-rules-permanently-on-linux/