The article above says not to plug 12V into the arduino jack socket.
No, it says not to try to run the 2 amps for the shield through the Arduino. Big difference.
You can connect 12V to the power jack, but you need to connect 5V to the cellular shield. Just recognize that you need two connections to the power supply, at different voltages. Or, you need to build a separate 5V regulator to drop the 12V to 5V for the shield.
The 5V regulator on the Arduino will not handle 2A.