Two factor authentication

Hi I have an Apple ID which was previously singed in to some of my devices including an iPhone 6 which was stolen now every time I try to log in to my Apple ID from my new phone I am asked the password of that iPhone 6. Now I don’t remember that. The problem is I’ve activated two factor authentication also but can’t turn it off. The option is grey in my new phone and in other devices too. How do I turn it off please help.