It only uses up GB if you have cellular data on?

I have an iPhone 6s and I have data on my phone. And I have my Cellular data turned off but I keep getting texts saying I'm using to much GB? What am I doing wrong to make me use it up? Please help. I keep getting yelled at because of it.

1. Make sure your home Wi-Fi signal is strong and reliable (some parts of the house may not be close enough to the base station)
2. Turn off Wi-Fi Assist. That feature backs up a flaky Wi-Fi signal with cellular data.
3. Check the list of apps allowed to use cellular data, and see which ones are consuming the most.
4. If you're on a family plan with shared data, you may not be the only culprit. Have a polite discussion with your folks about the family usage to narrow down what else could be using data.

Honestly I wish I had an explanation, but if you have Cellular Data turned off then you shouldn't be using any data. Could download a data monitoring app & see if anything is being used & if so I would alert it to an Apple Store because that is not expected behaviour under those circumstances