I'm using the same iphone model and the same iOS version, and everything is fine.
Since you've already tried to restart your device (to no avail), I would suggest to restore your iphone to factory settings, so that you install a fresh copy of the latest iOS (11.1.1).
To do this, make a full backup of your device, then follow the steps provided in this support guide, setting your iP7 as new.:
Then restore from your previous backup.