Special offer in contract was not fulfilled

Hi,
I made my contract last October 2019, I can’t speak german, so I bought my current offer “O2 M boost” from an O2 store in Berlin … and the agent explained the offer in English and it was supposed to have 6 months free of payment … and then 34.99/ month. The agent I bought from my line told me that there will be 39.99 one time payment then 6 months without payment then 34.99/ month
I traveled outside gremany since January and then got stuck abroad with the COVID-19 and just came to Germany last week on mid June to know that i’ve been charged for all the months from the start of the conract.

My question is, was there a 6 months free of charge or that was not included from the beginning ? ... also since I don’t speak german … the agent I bought from the line in the O2 store .. wrote to me my plan on the back of the contract I’ve signed … then he signed it and stamped it…. so I’m not sure why it was not included.

attached is the page the agent wrote with his hand on the back of my contract, signed it and stamped it.