DND mutes all notifications and vibrations only. If that isn't what you want, use the silence notifications option as mentioned eaarlier, rather than DND.
There is a further alternative available, hidden in the settings menu...
Mute all sounds, including earpiece, so you wont hear anything even during a call...Just like there is no speaker or sounder. The phone is silent in every way.
Maybe this option will meet your needs. On the Samsung S8 you go to Settings/Accessibility/Hearing Enhancements/Mute all sounds. There is no indication on the main screen you have selected this, so don't forget to re-enable it if you want to use your phone later for a call!