Dear friend, greetings to Moscow, you can try an helper application, which can make game controllers like x360 or any joystick (I guess) to qwerty keyboards or mouse events. Then you can map anywhere else, any software, including Reaper, as you wish. Here the program: antimicroX.
https://github.com/juliagoda/antimicroX I am using a version for linux. But you can check also under antimicro for other systems, it should work. Good luck. Do not forget to report back here if it worked. Have fun making music.