SOLVED: Soft Power On SIM900 module - JP pads missing!
OK for anyone who has the same problem. I solved this by soldering a cable on the underside of the board on this pin (see image) on power button. Then I connect the cable to pin 9 on Arduino and run this on sketch:    

  pinMode(9, OUTPUT); 
  digitalWrite(9,LOW);
  delay(1000);
  digitalWrite(9,HIGH);
  delay(2000);
  pinMode(9, INPUT);


I don't know how it works, but it solves my problem.