I recently and for the 1st time ever logged out of iCloud. I now want to log back in. However, after I have entered my  Apple ID and password, it eventually asks me for the 6-digit number used to obtain access to my iPhone 11.  I have for many  years, and certainly from  long before I owned the iPhone 11, used the same number.  iCloud does not recognise this number.  Where do I go from here ?