In the old days of straight connection of a MODEM to a computer's serial port, the computer was directly behind the IP address, so if someone wanted to connect to say, 45.127.33.240 they are guaranteed to get ONE computer behind it.
Nowadays most MODEMs are built into wireless router/switches. This requires a technology called NAT to allow connections between ONE computer and the outside world. It doesn't allow some random computer to connect to it's IP address and get ONE computer behind it. Port forwarding sets which ONE computer will be seen if a computer tries to connect on that port.
In re your assumption that you cannot portforward with your Vodafone Mobile Broadband USB adaptor. No. You can't portforward on it. If you're lucky, Vodafone gave you a dedicated IP address. If not, the NAT is going on at Vodafone itself. In which case you cannot control it.