That "T-Zero" that was displayed was not a change to your network.
In iOS 11 when you switch out of an app(without closing it from the multitasking screen) it will show that app in the top left corner where your network would normally be displayed.
If you click on the T-Zero up the top left it will bring you back to that app. It's there for quick multitasking, it does not affect your network settings at all. It just covers the area where your signal would be displayed. If you close the app(from the multitasking screen) your network status will be displayed in that area again.
Apps cannot change your network settings like that unless you're using a jailbroken phone.