Update: was able to do a chat with Apple support. Although I had tried the reboot myself based on Internet search, I had not done exactly right.
The correct way is to depress the volume up button on the left quickly, depress the volume down button on the left quickly, and then hold the power button on the right for a minimum of 30 seconds or until the Apple logo appears (I did not hold it down for long enough when I first tried this).
Also, the first time I tried this with Apple support it did not work, but then I immediately tried it a second time and it did work. The Apple support person told me he has needed to do this with his own phone and it often takes two or more attempts before it works.