calibrate compass

I've had the P4p v2 for two weeks. And it's flown really well. Shot 6 jobs already. It's so stable. When I need to do a tracking shot to the side, it goes exactly in line. No drifting. Mostly. But today after maybe 20 flights, when I started up, the compass symbol was facing the opposite...

So, I crashed my Phantom 3 Advanced and now the gimbal makes the normal sounds that it makes when it's moving, but doesn't move. The sound repeats as long as the power is on. On the screen on my phone it tells me that there is a compass error and to calibrate. All attempts to calibrate by...