You can turn the oven off from the app, but the issue is that there’s a button on the stove that you must physically press each and every time that you want to be able to use that feature. Every time.
After you close the door, press Smart Control on your range’s display to activate the feature
So in the OP’s case, he would have had to push that button when he started cooking in order to be able to turn the oven off in the app later.
AND—you still can’t turn off the burners on the top of the stove! 
just the oven (the part with the door.)
I have no idea why not. A Samsung employee posting in the following thread said it’s for safety reasons, which I could see for turning the burners on, but not for turning them off.
Here’s a previous discussion with more details, but still no solution for Samsung models.