I had connection problems with the Pixhawk 6c too. In my case, I was able to solve them by setting
- 
MAV_1_RADIO_CTLtodisabledand - 
MAV_1_FLOW_CTRLtoforce off 
See here for the complete description.
I had connection problems with the Pixhawk 6c too. In my case, I was able to solve them by setting
MAV_1_RADIO_CTL to disabled andMAV_1_FLOW_CTRL to force off
See here for the complete description.