AT Command for SIM5320E

I've connected the arduino Mega with my SIM5320E as below:

SIM5320E <--> Power Source / Arduino Mega 2560

5V <--> 5V (Power source)
GND <--> GND (Power source)
VMCU <--> NOT CONNECTED
STXD <--> RX217 Arduino
SRXD <--> TX216 Arduino
GND <--> GND

But I am unable to send AT command successfully.
Is there anyone able to advise on this?