GMX account?

Hello, I can no longer access my GMX account. Apparently, my password was compromised. The system has to send me a code to get a new password. I can't do it by email anymore. My phone number is no longer valid. Where can I enter the current one? Or how else can I access my account? GMX sent me various links, but they all ask for login credentials that are no longer accepted. Phone service is subject to a fee. What should I do? I have important data saved in the cloud and would really like to remove it. Thanks for your answers!