How to change the default volume?

Hello folks,

I've been using an iPhone SE for a few months with iOS 11 and stumbled upon a strange omission: I haven't found a way (at least any obvious one) to change the default volume. So every time the phone exits the Do not disturb mode, it sets the volume at about 12 bars out of 16 - you can imagine what a "terrific" experience this is when any notification blares next to my head in the morning! What have I been missing, does anybody know how to tell the "smart" phone that I'd like to be in charge of the basic features? Thanks for any remark...