3DR not working at first time

Hello everyone,

My kid has a school project that using pixhawk and 3dr device for telemetry.
Before we paired PC to PC both 3DR by USB connection they working and all LEDs goes to solid green.
But telemetry not pairing at first time when pixhawk powered on, so when we disconnect and connect (unplug and plug) from the terminal of “TELEM 1” it is starting to communicate to PC’s 3DR.

So actually I didn’t understand this issue.
Do you know anyone this kind problem and solutions?

Best regards