you can cancel there: https://info.o2online.de/kuendigung/   
Your new provider commissions the RNM at the end of the contract.

​@Bumer If i cancel the contract will i still have my phone number assigned to me? Cause i have used them in alot of documents verification 

only if you sign a new contract somewhere else and take your phone number with you.

​@Bumer so i have T-mobile sim and my o2 phone contract ends by next month but i don’t want to lose the number because i have them on documents that are importance including work and online stuffs. So please explain to me like a 2year old How i can go about it? Because the network is not really good in my area and i need to go for t-mobile internet.

Hello ​@Tunjesky ,
welcome to our o2 Community 
Would you like to read this article? 

Just let me know if you have any questions. 
Kind regards,
Solveig 

​@Tunjesky 

is the T-Mobile contract already active? Telekom does not offer subsequent phone number portability.

Hello ​@Tunjesky 
It's a shame that you don't have sufficient network coverage with us and therefore want to cancel 
I wanted to ask you, did you get all the information from us here about canceling and transferring the phone number? Have you already been able to order this?
Best regards

Kathi