Drop your iptables.rules into /etc/iptables.rules
Create service template like so:

sudo nano /etc/systemd/system/restore-iptables-rules.service
Copy-paste this:

[Unit]
Description = Apply iptables rules

[Service]
Type=oneshot
ExecStart=/bin/sh -c 'iptables-restore < /etc/iptables.rules'

[Install]
WantedBy=network-pre.target
Enable service like so:

sudo systemctl enable restore-iptables-rules.service
Reboot and check that the rules have been applied:

sudo iptables -L