Why do you want to send the device back after 2 years?

I had forgotten where it was. I assumed I had sent it already when I cancelled the contract.

Unless you have received an invoice for compensation for unreturned rental devices in the last 3 months, I would recommend simply keeping the device. 
If you no longer have any use for the device, you can check whether it is still possible to return it via the returns portal. 

Hello @Sourabh Parkala,

I assume that your router no longer needs to be returned after two years, it will have long since been written off.

You can check it here by entering the serial number: Router Rücksendeportal. You can find the serial number on the sticker under the router.

Please let us know if you have any further questions.

Best regards

Giulia