I have found in my limited driving of this truck (which I believe was built on a Friday due to the issues I have with it / Reverse only with Diff brain connected/ broken rear bumper hoop/ cracked- broken rim) To get in reverse you need to go Full Brake> Neutral> Then Reverse ( at least that's what works for me anyway)
For your reverse that is slow speed- the 2 buttons on the grip of the transmitter/ controller will change that if you like> top little square button is the "+" for increased speed/ Bottom little square button is the "-" for slower speed (Page 19 in the manual- end point adjustment buttons) I can only get them to work for reverse, there is no change for forward in my testing.