Cancellation of monthly (Home flex 100 M) internet contract

Hi, so I just got my monthly internet contract activated on 23.1 (O2 Home M 100 Flex). I am currently staying in my sublease until 31.4, and I know that I won’t be needing the internet contract afterwards

I was told by the english customer service that the monthly bill starts according to the activation date. For example my january internet bill is from 23.1-23.2. Since I want my internet to last until 31.4, it would be exactly 3 months ( 23.1- 23.2 and 23.2-23.3 and 23.3-23.4) and 8 days. Does this mean that If I want my contract to end on 31.4, I would need to pay 4 months of bill or 3 months and ¼ of 1 month bill , or I just have to end the contract on 23.4 and live 8 days without internet?

In addition to this, If I want the contract to end on 31.4. What is the earliest day that I can send the cancellation mail, and is the possible to do this online instead of sending mail?