Once you have set up hotspot you do not need to go into the settings to turn it on.
To turn on hotspot swipe down from the top right of your screen on an iPhone X, or if your phone has a home button (iPhone 8, 7, 6 etc) swipe from bottom to top to open the control centre.
Next, tap and hold inside the top left square to open the network options, where you will find Personal Hotspot.
When you turn on hotspot it will turn green and your WIFI will disconnect (only if it is connected to another network).
You'll need to turn Personal Hotspot on any time you restart your phone, or if you have been away from the terminal for more than 5 minutes.