Welcome to the Forums @Geline46!
This might be happening because the OPPO R11s is not on the supported devices list. This means that the app has not been fully optimized to work with that phone and thus issues like this are to be expected.
As long as the phone fulfills the minimum requirements, Android 4.4 and Bluetooth 4.0, there is a chance that they will work correctly however we cannot guarantee it. Having said that, some users have mentioned that following these steps has helped them get their phones not on the list to sync and work correctly. Maybe you could give it a shot.
Let me know if you have any further questions.
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