What is best approach for setting up internet gateway from orange pi running Raspbian

I have cheap IoT device from Orangepi running raspbian server with wlan0 and ppp0 interfaces. wlan0 will be connected to access point in the local network. the ppp0 interface will be connected to GSM network through /dev/modem0 and provide internet access for the device. 1. want orangePi to be able to connect internet itself through ppp0 2. want all devices in the local network that will be connected to router/access point be able to connect to the internet through IoT ppp0 interface 3. also want to be able to setup port forwarding from ppp0 interface to some devices in the local network What will be the best approach to do it and how to configure IoT Linux box?