Hi,
You say that you just installed an app (sidesync) and the problem started after that, is this correct?
Try using the phone in [https://support.t-mobile.com/docs/DOC-27712|safe mode].
This will help to show whether the problem is hardware related or software related.
If it works OK when in safe mode then a downloaded app is the problem. The trick is to find out which one. You may wish to uninstall sidesync and see if the problem disappears.