Unpaid invoice and contract termination

Hi

I'm a foreginer who unfortunatelly signed a long term contract with o2 (instead of prepaid). Had to move out from Germany after some time and stoped payig the bill (don't judge). I have a few invoices unpaid and my contract is almost over.

• Do I need to fill any form to cancel my contract 3 months before the end of it to avoid auto renewal?
• When I try to log in to Mein O2 I have an information that my contract is terminated and account closed. So I'm not sure about the status of the invoices.
• If my account was closed and terminated and my debt sold to 3rd party company how can I find out the status of that debt? I didn't recieve any email or letter by post.

Thank you for help