Catalina v10.15.x - IPv6 blocking IPv4 internet connection

I have an MBA running the latest release version of Catalina. The network has been stable and is running with other iOS and Windows devices without issue.

The MBA problem is that it does not connect to the Internet. The only way to clear it is to go into the advanced network settings, go to the IP tab and change the IPv6 setting to “local”. This gets the user back on line. However, this does not “fix” the problem, as it will return again as soon as the machine is rebooted or restarted.

There is no option to switch off IPv6 in Catalina, and nearly everything you read says that IPv6 is the way to go. Once again though, it is emphatically NOT the way to go if the ISP does not support it and if you are working with domestic users whose networks also do not use IPv6. Whether anyone likes it or not, IPv4 is still used in over 80% of the world, so the question is:

”How do you prevent IPv6 from hijacking IPv4 connections with macOS Catalina properly?”

To prevent unnecessary replies, I am a technical communications specialist with over 30 years’ experience with networks of all sizes and shapes, and have been with Apple products since the Apple II, so please reply (if possible) with that in mind.

My current answer is that this is probably an Apple issue that needs Catalina to be updated to permit IPv6 to be switched off rather than the “local ...” that only works temporarily. The Terminal networksetup command doesn’t not work to enable the switch off for Wi-fi either (used to work with earlier macOS versions). I’m out of ideas. Anyone else had this and fixed it?