@JTan Read this FAQ for this. In this FAQ is a link for the o2 return portal for rented routers  too.
 

Upload from your papers (official  deregistration form) you can upload here.

https://info.o2online.de/kuendigung/
 

Thank you for your response.

So I understood that I can send the modem back without the de-registration letter first.
Once I receive the de-registration letter,I will upload it through the link you shared?

@JTan You can at any time send back the o2 router. No extra information for o2 need. But again only use for this the o2 return portal. After the router delivered and checked by o2, it will been automatically booked out. But keep the DHL receipt!

https://router-retoure.o2online.de/start/app/dashboard

After getting the de-registration letter you can upload it. Remember what you must pay for one month after o2 confirmed the extraordinary cancel from your landline internet.

Don‘t cancel to quick your German bank account.

edited: more information added

Hello @JTan ,

welcome to the o2 Community .

As I can see you’ve already received a lot of help and infos from Joe Doe. Is there still anything else that you need help with, then please let us know.

Kind regards, Sven

Hi all, I did as per instructed. 
Have left the country for almost 2 months now, but my ex landlord in Germany received this letter. 
My German is not really good but does it says my contract is not activated again?
Please help me terminate them as I have left the country already. 
 

 @o2_Sven Hi, I received an invoice already for this. Please advice what I should do next?

Dear @JTan ,

your contract was deactivated on August 15th.

Your last invoice has now been created and is due this month. There are five days left to pay that fell into the current billing cycle.
Please pay the outstanding amount with your customer number to the bank account below.
Hypo Vereinsbank AG
IBAN: DE 16 7002 0270 0005 7131 53
BIC: HYVEDEMMXXX

Kind regards,

Andrea