Cancel O2 contract a few days after SIM card activation?

I just moved to Germany to study and i purchased online an iPhone 7 Plus with an O2 free M tarif, I paid the delivery and 170 € in advance for the phone and when i got the package there was two envelopes, I opened them and each of them had a SIM card, i thought one of the SIM cards was part of the MultiCard service so i can use it in other device with the same number so I saved it for later, almost one month later i got an e-mail telling that my SIMcard was activated and when i checked it was a totally different number than the one that i´m using right now with the cellphone. I went online to My O2 account and i realized that It was a different contract, that I don´t want, and now instead of paying 52,49 per month i have to add extra 34,99 for that SIM card.

I really don´t know what to do because when i call everything is in German and when i tried the Live Chat service they told that they can´t assist me in English.

I´d like to know hot to cancel this contract before June 31th because that´s when they will charge the extra amount to my debit card.

i read online that it is possible for customers that bought online to cancel their contracts but i´m not sure about that and if it´s possible i really would like to know how to do it.