Finally got it installed. This is what did it...
According to this https://www.computerworld.com/article/3199077/microsoft-windows/windows-10-a-guide-to-the-updates.html
The update progression from OS Build 16299.68 is via 16299.98 to 16299.125
So I manually updated using KB4051963 to OS Build 16299.98 and then manually installed KB4054517.
The router was turned off to stop the automatic updater which was trying to butt in.
Took a while but there were no further problems.
Don't know why KB4051963 was missing.
I would expect Cumulative updates to be designed to contain all previous update files. If that is the case then KB4054517 must be incomplete and is more like a "Delta" update.
This worked for me as well. Thanks! Damn thing wasted hours of my time. People having this issue should try this solution. Uninstall if any of these updates were already installed first.