xBawwRx
I use DND frequently and it works correctly.
Double check which mode is actually engaged when it should be running. Is it dnd or sleeping mode?
And then check those settings. There's not too many of them. You can change every setting to something else then back again (e.g. don't allow any calls/texts > only favorite contacts > don't allow any calls/texts)
Sometimes electronics need that. Off/on or disable/enable. A change in state.