Rewind function on live tv

For years, when I turned on my tv, I would have the option to rewind the current channel the tv was on, up to 1 hour back.  Great feature, especially for catching the news or other programming that had started before we came inside in the evening.  
Recently, I had a technician at my home checking my internet.  The Xfinity technician upgraded to a digital DVR.  We soon discovered the digital DVR did not have the Live TV Rewind feature we loved,  so we went to our local store and they gave us an older DVR which they said would have the rewind feature.  
It did, and worked for about 3 days.  Now not even the tech who came back to our house today, 4/24/26, could make it work, nor did he have any knowledge of the rewind feature or why it no longer works.

What is the deal with this feature.  Why does it work and then stop.  Why does it work in some areas and not others?  WHY DOSNT ANYONE AT XFINITY SEEM TO HAVE ANY KNOWLEDGE OF THIS FEATURE, ITS HISTORY, ITS FUTURE, (or lack there of)?