When you buy a sim subscription and add a mobile phone with a payment plan you have to obey the contract you signed. Even if you think it is too expensive for you later.
Odido can help you when you want to shorten the contract period after the contract has started. After one month cancellation period, while you pay 100%, the remaining months you pay 50% + the remaining phone payments. This will result in one end invoice and the sim will be diisconnected after the cancellation period.
If you all do not like this, than keep yourself to the contract and terminate at the end of the contract period.