@RandySea2
First of all, you asked me to record what happens on the browser on my smartphone. If you look at my question, you will see that I am talking about the browser on my laptop. I have never tried logging on to the Deco from the browser on my phone. The screen is too small to be useful for the Deco web UI.
However, this is now moot. Somehow, the Deco has self corrected, two days after the firmware update. I can log on to the web UI from my laptop normally, as I did before the upgrade.No idea why it's now working, as my logon is the same. Oddly, though, the main Deco started up after upgrade at IP 192.168.68.5 where previously it had been 192.168.68.1. This morning, the main Deco was back to 192.168.68.1.
A second oddity was that when I first did the upgrade, the signal sources were changed from a correct mesh (each Deco getting a signal from the closest other Deco) to each Deco getting its signal from the main Deco. This changed by itself since yesterday to an optimized network of Decos. It took 48 hours, but now it is working correctly. I should note that there are many postings about the problem the the Deco M5 will not optimize connections, so I guess I'm lucky.
Bottom line: For now, my problem has gone away by itself.