How do I go over data limit while data is switched to off on phone and on I'm using WiFi?

I hardly use my internet at all while I'm at home I'm always on my WiFi I recently just got a message saying I went over my cap while I was on my WiFi and the data was switch to off on my phone. How is this possible

Hi Michael. Are you using an iPhone? If you are, I'm thinking it's Wi-Fi Assist. It automatically turns on mobile data when Wi-Fi connectivitiy is poor. Open Settings > Mobile Data > turn off Wi-Fi Assist.

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