Thrust reverse is controlled by RealFlight Channel 12 in the Controller Profile.
Try pressing the button on the DXS. If you can go forward while the button is being pressed, it's a very small change to edit the Controller Profile to click the "Reverse" box for Channel 12. If that still doesn't help, then you'll need to get Channel 12 mapped to the button. Edit the Controller Profile, then find Channel 12, then click on the blue box on the right side for Channel 12. It will tell you to move the input on the controller that you want associated with that channel; press the button on the DXS at that time. That should make the connection. Save the Controller Profile.
