I too was receiving texts stating I was at 80% than 100% data usage then have now been charged $50. For overage. I haven't done anything different and actually thought the texts were a scam. Apparently not. Haven't been able to speak to anyone over the holidays. Something is definitely wrong I actually had my cellular data turned off and have been within a wifi area in my home since Dec 22. What the heck?
I know that no one likes to see Data Overage charges on their bills, it's a very unpleasant surprise! Especially if you've disabled the Cellular Data on your device.
Can you describe how you disabled the Cellular Data on your iPhone/iPad? Do you have the Wi-Fi Assist feature enabled or disabled? Have you done an update on the OS of your device recently? Have you ever noticed this type of unusual data usage previously?