How can I add iptables rules that persist across reboots?
Running scripts at boot (init) as you are trying to do is only supported if the kernel supports it.

If you are using a stock kernel, you may need to install a custom kernel that includes init support.

There are also apps that start with Android and automatically apply firewall/iptables rules, however keep in mind these require root access.

It is not possible to modify iptables rules without root access. However, you can use a VPN connection to route network traffic which works on any Android 4.0+ device without root access.