Trouble using mavros /rc/in topic with pixhawk

edit

I'm using a Herelink remote controller which is connected to pixhawk purple-cube via mavlink2 protocol on the telem-2 connector pin. I'm trying to read the rc/in topic in mavros (using rostopic echo). command used:

roslaunch mavros apm.launch fcu_url:=serial:///dev/ttyACM0:57600

This is what I can see:

{
header: 
  seq: 71
  stamp: 
    secs: 1606131722
    nsecs: 352759947
  frame_id: ''
rssi: 0
channels: []
}

As you can see, none of the 16 channels are visible. No changes in the rc/in channels array occur even with button/stick inputs from the herelink controller.

Do note that communication between the pixhawk and the computer is working well, I ran rostopic echo /mavros/imu/data and the appropriate data was being published and could be viewed.