Issues starting up my SIM800L

My first problem is that when i went through the datasheet of the SIM800l it says it requires a maximum voltage 4.2V. But each time i step down the voltage below 5v, the device does not come on. It only powers when the voltage is 5v. Please what can be the reason for this. I am powering my device using arduino uno.

Secondly, could this be the reason why my sim card does not communicate with GPRS but works with voice calls and sends SMS.

Which module ?
Arduino power may not be sufficient..

its a Sim800L and with and an Arduino UNO mega. Please do you know the minimum power expected for the SIM 800L