RaspberryPi to Fona tethering via a serial connection. It works on an Australian 3G network using a standard Amaysim SIM card from a mobile phone.
Use this tutorial at Adafruit to set up the tethering.
Use this tutorial to turn the Pi into a router using a DHCP server and NAT. I’ve just made it a cat5 router as in this situation I don’t need wifi, but the tutorial shows how to add a wifi port to the Pi. It’s just a matter of using
to check your port names and then choosing the right ones. For example using eth0 and ppp0 instead of wlan0 and eth0 for your inbound and outbound ports in the firewall rules.
To make it all work on boot you may want to add
to the end of