If Apple was not able to resolve this for you, then there is virtually no chance that we will be able to resolve it here as well. This is because Apple had the opportunity to examine your iPhone physically and by running diagnostics.
If you still want to resolve this issue by yourself, then read below:
(1) Try and Force Restart your iPhone EXACTLY as shown below and see whether that resolves the issue:
- Press and quickly release Volume UP button
- Press and quickly release Volume DOWN button
- Press and Hold the SIDE button until an Apple logo appears and then release the Side button (Can take up to 20 seconds. (DO NOT release Side Button when invited to Slide Power OFF).
(2) If the above does not work then - Follow the instructions in this article by Apple ➔ If your iPhone is Non-Responsive or Frozen and see if that helps.
Axel F.