Make iptables rules persistent after reboot on Ubuntu 18.04

sudo apt install iptables-persistent netfilter-persistent
systemctl enable netfilter-persistent
netfilter-persistent save
netfilter-persistent start

iptables-save  > /etc/iptables/rules.v4
ip6tables-save > /etc/iptables/rules.v6

iptables-restore  < /etc/iptables/rules.v4
ip6tables-restore < /etc/iptables/rules.v6

systemctl stop    netfilter-persistent
systemctl start   netfilter-persistent
systemctl restart netfilter-persistent

Remove persistent iptables rules

To remove persistent iptables rules simply open a relevant /etc/iptables/rules.v* file and delete lines containing all unwanted rules.