SIM800L not registering to network
I am 90% sure it's a power issue. Not sure on what board/module you're using but the SIM800L/SIM900 needs a burst of current of 2&nbsp;A during 577&nbsp;µS (See the manual). 

Try using 4700&nbsp;µF and a 10&nbsp;µF capacitor in parallel with the module. Powering it with a proper power source instead of the Arduino wouldn't hurt too.