Use this systemd service to have /etc/iptables.rules reloaded on each reboot:

In /etc/systemd/system/iptables-restore.service:

[Unit]
Description=Iptables service

[Service]
User=root
ExecStart=/sbin/iptables-restore -n /etc/iptables.rules

[Install]
WantedBy=multi-user.target

Run the following enable the service to run on boot:

sudo systemctl enable iptables-restore.service.

Saving IPTables Changes:

Run the following command to save your iptables rules to /etc/iptables.rules to be restored on each boot:

sudo iptables-save | sudo tee /etc/iptables.rules