OFFBOARD MODE with PX4_HITL, MAVROS and QGC

Hello everyone, I am working on testing model-based design control strategies in Simulink and then test them in px4_sitl and px4_hitl with Gazebo-Classic through OFFBOARD flight mode. The control input is sent to the drones via MAVROS topics, because in real world experiments we have Holybro X500 drones equipped with Pixhawk 6C and Raspberry Pi as onboard (on which MAVROS node will be running).

Regarding px4_sitl everything is working fine, I am able to control the drones, make them take off and everything in offboard. However, when doing the same with HITL (as described by the PX4 page and using the exact same Simulink model) the problem is QGroundControl keeps saying no offboard signal even if position or acceleration setpoints are being published. In the setpoint message each field is populated, namely timestamp, frame_id, etc and I can see the topic correctly published in /mavros/setpoint_accel/accel. The environment is an Ubuntu Linux machine with version 20.04 and ROS Noetic. Mavros runs on board the RPi and the communication to the FCU is done through USB to TELEM2.

Thanks,
Elia