Firmware 12.1.11.10 iPhone disconnects

iOS 26.6 on an iPhone 17 Pro

I updated via the app and it seemed to go smoothly - all the devices showed the correct FW.  I have made no changes to the setup other than that update.

IPv6 is on the router.  Channels are 11 and 36.

I have the IoT set to 2.4 GHz and 5 GHz WPA2-PSK and my phone is happy with that and connects OK.

It's the main network that is flakey using WPA3-Personal.  For example, right now my phone shows it as connected to the SSID, but the WIFI status at the top of the phone shows no network, only LTE.  Yet, my iPad sitting next to the phone is connected to the same main SSID and has no problems.  The phone is apparently connected to the router and the iPad to one of the satellites if the connection information is correct.

So, the phone can connect to the SSID but then acts like it has no WIFI even though is shows as connected to the SSID.  Power cycling the phone does no good. Rebooting the router fixes it for a while and then the problem recurs. Or, toggling OFF Wi-Fi 6E Mode on the iPhone - not what I want to do.  Or sometimes doing nothing but toggling between the two SSIDs (main and IoT) fixes it for a while.  I'd say it's random, but that just means I don't know what is causing it yet, so it seems random.

I have not had time to do much troubleshooting and am not sure where to look, other than going back to the previous firmware that had been fine.