How does Rover get back?

Rover : hpi savage flux.
firmware: Px4,
airframe: Traxxas stampede VXl 2WD.
Now push the throttle forward only, not back.The use of the pitching channel also can only move forward.

Reverse needs to be added to the mixer and default setup. See this PR for more details and discussion. https://github.com/PX4/Firmware/pull/8037