If Steam Support can’t verify the account, the best thing you can do is gather as much proof as possible: old purchase receipts (email or bank), the exact date you created your account, any CD keys activated, or your original email and past phone numbers. Even your last few login IPs or devices (if remembered) can help.

Without the recovery code or access to the original phone number, Steam has to be extra strict for security reasons. Keep responding to the same support ticket with more info starting new ones will slow things down. It's frustrating, but persistence usually helps.