Try a forced restart: Press and hold both the Home and the Top (or Side) buttons for at least 10 seconds, until you see the Apple logo.
I this doesn't help, and since you have already tried to reset network settings to no avail, make a full backup of your iphone and try to install a fresh copy of your iOS, restoring your iPhone to factory settings, then set your device as new (follow the steps here provided):
Restore your iPhone, iPad, or iPod to factory settings - Apple Support
Then restore it from your previously made backup.
In nothing helps, contact your nearest Apple Store and have your device inspected.
Regards
Giulio