Buying a new phone

at o2 the phone is an independent contract. You can complete one at any time.

https://www.o2online.de/e-shop/handys-ohne-vertrag?ohne-tarif=ja&zielgruppe=bestandskunden

Thank you, for your answer. 
I want to understand what I can do to enjoy a top deal? Is it still possible? I wasn't offered nothing by store. 
If not possible to add the mobile (top deal) in this contract, is it possible to cancel my contract and do another one? 

There is no discount on *my Handy* contracts. You got it in the shop when you renewed your contract.
Cancellation, if possible at all, can only be done in the shop where it was completed. It would be pure goodwill.

I don’t quite understand why if you wanted a new phone you didn’t ask for one when you extended the contract. As already stated, when you extended the contract without the phone you probably received a discount on the plan you chose, right? Then it doesn#t make any difference if you buy the phone separately now.

There is no cancellation right for contracts concluded in a shop, so you would have to ask the shop if they are prepared to cancel it as a gesture of goodwill.

Well, the person attending to me didn't speak English and I had to do everything with my A2 German (under construction). Not the best situation.

 I got 10€ discount while the mobile without contract would cost 60€, same amount of the top deal. To get the mobile now it would cost to me 87€ at the end of the month. Not the best deal. 
I’ll talk to the store again, but not sure if it will be worth it. 

Thank you for your time