I have the exactly same issue you spoke. I fly frequently using the Radar and if I keep the iPhone 7 Plus with home button to left side, the directions displayed become wrong.
Is this an issue with interference from RC to Mobile phone compass ?
The controller gimbals (control sticks) have powerful magnets on them. These are what messes up the compass on your phone. Try sliding the phone away from controller so that only the corners are in the holder. Don't use connector bracket that slides. Remove it and just plug cable intro phone so you have more flexibility in sliding phone out. You can also try turning phone around 180 degrees, or use a mount that lifts phone above controller.