That's bizarre that your S21 Ultra doesn't have the Mobile HotSpot Toggle in the QS Tiles.
Did you check by pulling down the Notification Panel all the way & then Click on "Edit Buttons" to see if the Mobile HotSpot is available, but just not added to your QS Tiles?
adb (Android Device Bridge) allows you to use adb commands to add items to your QS Tiles, among other things.
But it makes no sense that it's available on my S20 Ultra & not on your S21 Ultra.. if that is in fact the case.
My S20 Ultra Snapdragon variant is From Rogers SM-G988W.
For years it was believed that the Canadian Carriers also disabled the Mobile Data Toggle.. Samsung even blamed them, until Samsung Canada finally admitted that it wasn't the Canadian Carriers, it was in fact, Samsung Canada.
If you want, I can tell you how to add the HotSpot QS Tile to your device?
You need a computer, Samsung's USB Drivers installed on your computer, Platform-Tools installed on your computer, and run a simple adb command to add the HotSpot QS Tile.
But I really don't understand why you wouldn't have it already....🤔
Used to enable it in previous devices via ADB but it's pretty easy to get to via settings.