...
We have found that Google Gmail or Apple Mail works well. We have encountered issues when using the built-in Android mail app, where syncing your work and personal calendars has been unsuccessful. We are therefore unable to support this type of issue.
If the User Has A New Personal Device
Go to Users
Search for User & Find user
Click on Authentication Methods
Find old Device, click on Three dots & Delete
Then Click Require Re-register MFA
Then it's just standard, go to Web Mail, it'll show 'more information required' and you can setup MFA as above