iPad prepaid number is invalid

I used to have prepaid data on my old iPad but I got a new iPad and I put the same SIM card from my old iPad into my new iPad and I use it for Cellular now instead of prepaid cellular so I don’t know if that means anything but I still get charged for the prepaid on my old iPad and when I try to log into my account to cancel it the number which is (it might be private info so I don’t want to say it to a public message board) it says the number is invalid (my new iPad has a different cellular number) I’m trying to change the password with the number because I don’t know the password. So I can’t really do much and I just keep getting charged which is bit annoying and I want to get it fixed. The cellular on my old iPad says expired but I still get charged. I would not like to call the AT&T number because the way you get treated on there is not the best so if you try to help me don’t give me a number unless it’s not AT&T.