On my mother's Raspberry Pi the internet is accessed using a Mifi dongle which is tethered to the Pi using the USB cable.
Once the internet connection was working from the Pi itself I was able to share the internet using the following commands.
iptables --flush -t nat # Now we will do Masquerading ie. we are doing NAT. iptables --table nat --append POSTROUTING --out-interface wwan0 -j MASQUERADE iptables --append FORWARD --in-interface eth0 -j ACCEPT echo 1 > /proc/sys/net/ipv4/ip_forward
Note: The above instructions could be added to a script file.