Thanks for joining the Community @Simhard.
It could be because the P20 is not currently on the supported devices list. This means that the app has not been fully tested and optimized to work with that phone and thus issues like this may arise.
While the app can work as long as the phone meets the minimum requirements, Android 4.4 and Bluetooth 4.0, there is a possibility that some issues will happen. Some users of phones not on that list have mentioned that going through these troubleshooting steps helped them, so it's definitely worth a try.
Let me know if you have any further questions.
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