There is no need to pay before trying if your tx is supported. That's what the free demo version is for, so you can try if it works on your setup without spending any money.
If you get back with more information maybe I can be of help to get it working. If you wish to request a refund, please contact the staff at Itch, they handle all purchases.
I think it should be possible to connect the DJI FPV remote controller directly via USB. If it shows up as a USB HID controller in your operating system chances are Freerider will be able to read the input of the controller and calibrate it correctly. However I don't own a DJI FPV controller myself so I haven't tried it. Hopefully someone who owns it will be able to chime in.
