EAX20 setup failed, now my "Setup Device" won't work on the network I was extending

The short version: I unpacked my new EAX20, powered it on, and connected a Windows XP laptop (my Setup Device) to the EAX20 network and started the setup procedure. All went well. I selected to extend the networks (2.4 and 5 GHz) of my main modem-router (Sagemon FAST5355) and to keep the same SSIDs. It put up the page with the new credentials, finishing with the line

"To continue the installation process, connect your device to one of the above networks, select this check box, and click the CONTINUE button."

I disconnected the laptop from NETGEAR_EXT and connected to the main network that was to be extended. (NOTE: connecting to that network took a LONG time, but eventually it said it was connected.) I then clicked the checkbox, then the CONTINUE button. The browser address went to "www.mywifiext.com/IA_review_setting.htm", but it never loaded, and failed with "Server not found".

BUT here's the problem: With the EAX20 unplugged and packed away for the moment, the laptop that I used CAN NO LONGER CONNECT TO THE NETWORK I WAS TRYING TO EXTEND. It will connect to and function normally on other networks on my system, just not the one that I was trying to extend. (Actually, it will go through the same VERY LONG connection process, but then fails to pass any traffic for me. The web browser fails, "My network places"  fails, etc. Other devices in the house (phones, tablets, laptops, etc) are unaffected, and continue to function normally on that network.

I will sort out the EAX20 later (RESET and start over, I expect), but I need to get the laptop working again on the original network.

A network scan on the main system (Windows 7) finds everything as normal, including seeing the laptop come and go as I connect/disconnect it from that network, so it seems like some (?) permissions, somewhere, got screwed up during the installation.

Help me, please ....