Select users cannot setup Exchange mailbox on mobile devices.

Newly installed Exchange 2016 server.  Have 2 users with issues trying to get email accounts setup on their mobile devices.  Thus far I have looked into one of the accounts.  Both users are out of state so I don't have physical access to their mobile devices.  However, I tried to setup their Exchange account on my personal smart phone (Android) and using two different apps I am unable to get the account to authenticate and add to the device.  The remote users has an iPhone.  Supposedly they can add the account, but nothing ever syncs.  However, we have many other users in the organization that have successfully setup their accounts no their mobile devices.  In fact, we have attempted to setup another users account on the iPhone in question and that account works fine.  So I would say the users device is not the issue.  In addition, I am able to login to the users account using OWA with the same credentials and the user has Outlook 2016 on their computer using the same credentials.  Not sure where to start in trying to diagnose why it is not working.  

Help is appreciated.