@David-TP OK, I finally got this to work. I found that I got differing results depending on which of the 3 RJ45 ports I used on the main node. So, I switched over the main node with one of my...

@David-TP I have to say, hours and hours of my time testing all this could have been saved if TP-Link offered even the most basic of troubleshooting tools to the app, like easy and proper backup and...

@klipse I do appreciate your attempt to be helpful, but configuring a standard router successfully is not my problem.

@YorkshireBill Many thanks for your kind assistance, but I've already been through your set-up sequence many times (starting with a factory reset, and get it all working wth IPv4 first, etc.). The...

@David-TP Right, I've wasted a few more hours reproducing the errors. I can get as far as configuring Ipv6. The app says everything is OK, and displays what look like suitable connection data for...

@cocofyn Yes, I am using the Android app v3.9.42. Yes, those are the settings I used (and also various other settings as alternative attempts). I also tried entering the same user creds as used for...