Dear @QuelQuel ,
at first to your questions from the private message.
Our phones are not locked, you can use another prividers SIM-card without problems.
The mobile phone contracts usually run for 24 months. There are also contracts that can be terminated monthly (Flex), but these cost 5 euros more per month than the comparable 24-month contract. However, you cannot change from a 24-month contract to a monthly contract. In that case, you would have to cancel the fixed-term contract and sign a new one, in which case you would have to pay a one-time connection fee again and you cannot simply take the number with you.
The MyHandy contracts run for 12, 24, 36 or 48 months, depending on what you have in mind. It is also possible to pay off an O2 MyHandy contract early. In the end, however, it makes no difference to the final price.
I've picked out an offer for you with an iPhone 11 with an increased down payment for the phone, so the monthly rate is a little lower. It would cost even more with the iPhone 12. 
Are you considering an other manufacturer than Apple? We have some affordable mobile phones on offer. You can take a look here to see what comes into question for you. If the mobile phone is priced at a maximum of 16 euros per month, you will be in your desired price range.
Please pay attention, the final price is at the bottom of the offer.
If you like the offer, you can order it directly via the link in the email, simply click on the button in the email " Vertrag verlängern".
In accordance with the Transparency Ordinance, I would like to refer you to our product information sheets, which you can download at www.o2.de/produktinformationsblatt . There you will find all product details listed again (without discounts). There you will find all product details listed again (without discounts).
Kind regards,
Andrea