The next step.....if you have not already done this.....would be to perform a Forced Restart on your phone. This is the first troubleshooting step that a technician will do if you are taking the phone for service, so you might be able to save yourself a trip to the shop.
Force restart iPhoneApple SupportOfficial Apple Support › guide › iphone › ios
Quickly press and release the Up Button
Quickly press and release the Down Button
Hold in the Side button for at least 20 seconds or more until you see the Apple logo appear in the center of the screen. (Ignore the "slider" and keep holding the Side button)
If the Forced Restart does not help after a few tries, then this is an indication of a hardware issue on your phone.
Make an appointment at the Apple Store / Genius Bar or take the phone in to an authorized Apple service shop.
You can find a list of authorized shops in your area using this link:
Find Locations