Auto bed levelling with Rumba board - Servo not deploying
For future reference.

My issue about the servo not moving was caused by a wiring mistake.
The Exp. 3 has 14 pins has per this diagram.



However when phisically looking at the board, what you see is this:


I took the first 2 pins on the right of such connector and the 3rd one of the first row thinking that I was connecting pins 2-4-5 of Exp. 3.
I was wrong, because the first 2 (1-2) pins are not part of Exp. 3.

The right way to connect the servo is as following:


Then use PWM1 (pin 5 Ext.3)
I decided to leave trace of this issue and the relevant solution for someone that may experience the same issue.