Syncing Android 'Contacts' with an Office 365 mail account

My Android contacts used to sync with my Office 365 (exchange server) mail account, but this stopped working. More details: I have a Samsung S10 phone running Android 11 (up to date as of this posting). I use the native 'phone' app (green background, white phone symbol) which in turn uses the native 'contacts' app (orange background, white 'person' symbol, version 12.1.20.11). Up until recently, after making a call to a new number, I could choose to 'create contact' from the 'recents' list, and that name / number would automatically show up in my Office365 account online and in my windows 10 outlook installation within minutes. Similarly, contacts added in Outlook on my laptop would show up in the Contacts list on the phone within minutes. But recently, that stopped happening. I spent days researching, experimenting, and finally today I got it working again, but I'd like to see if others agree with my conclusions and also, provide info for others who may have run into this. (See my answer posted below).