On my Note 10+ and my wife's S10, the ability to turn on notifications for Google Duo is turned off. I"m not sure if this is a Google, Samsung, or AT&T problem.
Mine is grayed out in the on position. So your notifications options are off and grayed out? you can click the words of a notification category for more options, even if its on/off toggle is grayed out.
Better news, some of the other trouble shooting items I tried worked last night. I just assumed the fact it was "grayed out" was the issue, but it turns out it was other settings.
I took Duo off the list for battery saver (or battery optimization) and turned off data saving (I'm unlimited anyway...and spend 90% of my time at home on WiFi due to Covid WFH) and stopped allowing the phone to put idle apps to sleep. My wife and I called each other and we got the notifications!
