Hey Julianne,
There should be a mobile data toggle in the notification bar. It should look something like the third post here: https://forums.androidcentral.com/samsung-galaxy-s8-and-s8-plus/807460-does-your-s8-have-data-off-toggle-shortcut.html
Some S8's do not have this toggle, and can't use the notification bar switch. If this is the case for you, you can try a data toggle widget from the Play Store.