@Bernd_G
You will need to sign in as a separate global administrator user in order to disable MFA on your existing account. If you only have one global administrator account, you'll need to sign in as a regular user and log a support request with Microsoft to have them help you elevate that user to a global admin role. This process is typically (and surprisingly) fairly painless - You may be required to prove you own the tenant by, say, adding a TXT record to your Office365 custom domain, but it shouldn't take more than one phone call/20 minutes.
You can then use this newly elevated user to disable MFA on your existing account. Once you've disabled MFA, you can go here;
https://account.activedirectory.windowsazure.com/Proofup.aspx
And configure the authenticator app on your phone. You can then enable MFA again on your primary admin account.
