Hi there, cynthiakuntz.
I first want to say congrats on the new device. That is a beautiful phone, and I want you to be able to enjoy it to its fullest. Data management is definitely important, and I know how alarming it can be to get alerts like that. With the newest software version, Apple introduced a new feature called Wi-Fi Assist. The feature automatically switches between Wi-Fi and Cellular data, defaulting to the one with the strongest/fastest connection. To turn this off, you can go into settings> Cellular> Wi-Fi Assist> Turn it off.
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