There are known compatibility problems with a few mobile mail apps and Duo. Below is a list of apps that are compatible as of January 2021.
Androids: Outlook
iPhone iOS 11.x or greater: Mail or Outlook
iPhone iOS 10.x or less: Webmail
If you have a compatible mail app and the account stops working after you begin using Duo, you just need to remove and re-add the account to the mail app.