about my mobile phone contract cancellation

Hello, I already left Germany since March 2020 and will not return again. So I ask O2 to cancel the contract for me. I have questions about the following requirements:

• a deregistration certificate of your present place of residence in Germany
• a notification of your new foreign domicile
• alternatively a confirmation of your employer or landlord
• your approval to deduct three basic charges in advance

Q1. I don’t have the deregistration certificate of my place of residence since I left a long time ago, what am I supposed to do?

Q2. I moved back to my home in China, and I don’t know what is ‘a notification of your new foreign domicile’, is it like a photocopy of my identity card with my Chinese address on it? Or is it the passport with stamp on it that prove I returned to China in March 2020?

Q3. Can I use fax instead of sending physical mail since nowadays international physical mails are very expensive and slow?

Q4. How to contact O2 in English on the phone as I don’t know how to speak in German? I tried to call for a few times, but everything is in German, and I don’t know how to connect to a staff who could speak English