This app was mentioned in 5 comments, with an average of 1.60 upvotes
So I googled the model number with "missing mobile data" and discovered that it's intentionally removed on these AT&T versions. I did find this app($1.48) which can make one for you though.
Real dumb that they'd intentionally remove that quick setting.
Try this app: https://play.google.com/store/apps/details?id=com.xpe.app.quicksettings.manager&hl=en
It worked on my note 4. I am still waiting for my s8.
You could also try this : https://www.xda-developers.com/three-actually-hidden-non-clickbaity-tricks-for-your-new-galaxy-s8-or-s8/
There is an app for that.
https://play.google.com/store/apps/details?id=com.xpe.app.quicksettings.manager