ERNWOL8: Thanks for the update. I had already considered that possibility. However, I am certainly not buying a new unit to replace something that still works. I am not yet done with them. I will try again updating when my bandwidth cycle begins anew (2 days) and see what happens. Then I will contact them again. I have no doubt they couldn't answer those questions. I will not stop at the bottom of the tech hierarchy. I intend to drive this issue up as far as I am able. Any information I obtain will be posted here. It might be a time, but what the heck, I'm retired. 
Take care.
ERNWOLD8:
Greetings. Yesterday I called Verizon regarding an unrelated issue. I just happened to get one of their managers. She was very helpful and ended up connecting me to a very helpful Tier 2 technician. While speaking with him, I addressed the update issue. In a nutshell, this is what he said.
*This is a known issue with "some" of the AC791L's.
*Verizon has been communicating with Netgear about this issue since January 2018.
*They do not have access to the update file.
*The current situation is to either get another device or to ignore the update. He suggested a 7730L.
So, I was told essentially what you were told.
I am going to continue attempting to update the device. Perhaps the cosmic tumblers will align in time. It could be a simple glitch somewhere that may eventually be discovered and corrected. Verizon is definitely not immune from this sort of thing.
Since I purchase my devices and do not contract services, I will not be changing devices anytime soon. Unless of course the one I have dies.
While this may not be of interest to any of you, he also told me Verizon will be pulling the plug on 1X and 3G on 31 December 2019. Unfortunately it will effect me.
If I have any more news, I will post it here.
Many thanks and take care.