Cancelling a Prepaid Contract (US) from Germany

I have a problem: For my holiday in the US I got a prepaid SIM card from AT&T. In the store I explicitly said I wanted it for only one month and the saleswoman explicitly said they won't charge me anything after that. Well, now I did get charged another month, because they apparently forgot to cancel autopay. Following the homepage, I wanted to login into my account, but I need to enter a 4 digit PIN that I get via SMS. But I don't get any carrier here in Germany on the AT&T SIM card, so I can't receive any SMS.
What can I do now? I cannot find a contact email anywhere, for costumer chat I would need a login. They have my email, but if they ever sent one, it must have gotten into my spam filter (I did not think I'd need anything from AT&T).
My credit card provider can neither block AT&T from charging me, nor reverse the charge.
Can you guys help me find a way to cancel my contract? If possible I want to avoid paying the money to call the hotline from Germany.
Best Answer
The best way to cancel the contract is to call them personally from Germany and provide all the related info about your self. There is no any effective way to cancel such a thing rather than calling the hotline.
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