DSL Contract Cancellation Moving Homeland

Hi, 

I am moving back to homeland due to end of the assignment in Germany. 

I have normal contract until March 2021. 

I am moving in September 2020. 

I have to terminate the contract.

I gave online notice and gave call center. 

Call center said, I have to send some letter to O2 with some additional documents. 

I wont have any registration documents in India, Its my homeland. 

I wont have De-registration right now as I am moving in September and I will get such document only in September just before leaving. 

Can you describe what shall I do ?