Controlling a Gripper with Mavros

Hi all.

I am trying to control a gripper ( EPM Opengrab v3) by using Mavros. The EPM is connected to the PixHawk on AUX 1 header. The rail is powered independently of the PixHawk. Connecting directly to NuttX shell and using pwm app works as expected. Also configuring the parameter RC_MAP_AUX1=channel 8 works as expected. Note I did turn off the safety switch ( otherwise the above two methods would not work).

But the problem, starts when I try doing the same with Mavros. I first set the FCU to OFFBOARD mode and then publish to the topic /mavros/actuator_control the following message msg.group_mlx = 3; and controls[5]=1.0; But the EPM does not respond. Could someone please point out what is my mistake. Thank you.

NOTE: The EPM is connected to AUX 1 header on the PixHawk.

PX4 FIrmware: 1.5.5

I am using the same gripper but I use Arduino nano to activate the gripper. Arduino nano gets it from the companion computer.