What to do if you got a new phone and have MFA setup on the old one
Step 1: On your new mobile phone, open the app store and search for and download Microsoft Authenticator
Step 2: Login to portal.office.com in your web browser with your AST email and authenticate (if requested) with your old phone that has the Authenticator app installed.
Note: If you're unable to login to your account because you don't have or wiped the old device to authenticate the login, you can request a Temporary Access Pass and perform the steps to setup below the new mobile device.
Step 3: Click on your profile with your initials on the top right -> then click on 'View account'
Step 4: On the left, select 'Security info'
Step 5: Select '+Add sign-in method'
Step 6: Click the drop-down menu where it says 'Choose a method' -> then select 'Authenticator App' -> click 'Add'
Step 7: Follow through step 2.2 in this ARTICLE.
Step 8: Once you've setup MFA app on your new mobile device, return to the location in step 5 and delete the old device.