here http://www.electronoobs.com/eng_robotica_tut5_3.php it seems that I have to define in the firmware:
#define SERIAL_SUM_PPM THROTTLE,YAW,PITCH,ROLL,AUX1,AUX2,AUX3,AUX4,8,9,10 ,11
and instead of using UART I have to connect the receiver's PPM pin to the D2 pin the FC, I'll try