second card termination

Hallo, so I have two O2 sim cards. The first one is my main card with a 2-year contract which is due in December and the second one is the bonus card I pay for 5 euros per month because I also signed for the home DSL in May 2016. When I signed the home DSL contract, the salesperson told me that I could cancel the second card when I terminated my home DSL contract (which is basically anytime since it is a no-time contract). I went again to the O2 shop 2 months ago asking the same question, at that time the salesperson told me that the second card will be automatically canceled when I terminate my first card (because it is a bonus card). Now the time has come to terminate my first card, and now the salesperson told me that I can't terminate my second card because the contract is due in May 2018 and I signed for that. While it is true I did that, I was mistakenly informed by the previous salespersons. Back then I couldn't read German at all and I have no proof of the conversation between me and the salespersons. Is there anything that I should do? I don't want to pay for penalties and I do clearly think that is not fair.

Thank you.