To enable or disable Strong Authentication, please go to My Account > Login Details > Strong Authentication. On this page, you will be able to change your Strong Authentication settings and/or update your mobile number.
Start by selecting "Use strong authentication" and entering your mobile phone number, if it's missing.
After hitting Continue you'll be prompted to enter a 4 digit code.
Within a few moments, you should receive an SMS message providing the strong authentication code to confirm your change.
Upon entering the 4 digit code provided you will have successfully changed your strong authentication settings.