Definitely, bad power supply.
Try 12V. There is a step-down voltage regulator on board.
I feed 12V DC to my board (exactly same as yours), the switch is downside position (away from the nearest board edge )
And the marking says EXTERN (not EXT/ERN) 
You can provide either 6-12V external power via this connector, or plug 5V directly into board.
Do not forget to connect grounds if you use different power supplies to power Arduino and GSM