Long story short, you need to connect the radio in USB Joystick mode, which it sounds like you may have done already.
Then you have to spend time editing the Controller Profile in RealFlight to map the sticks and switches to the correct functions in RealFlight. RealFlight probably detected your TX12 as a Gamepad (as in Xbox gamepad), which is the default when it doesn't recognize a specific model of radio. It's not a problem at all - it just means you have to spend time making adjustments to build a correct Controller Profile.