Try resetting your password, its a bug on the iphones that you cannot get rid of. I have exactly the same on mine, although the Do Not Disturb is not turned on, under Silence it shows “While iPhone is locked”. You cannot get rid of it, I’ve tried. My friend has the same thing on her iphone. I found resetting my password helped and it rings now.
Turn your phone off by holding the button at right side. Swipe off. Restart the phone by pressing the side button again. After you have entered your password, use the buttons below the mute button on the left side to turn the ringer up.