Airport Utility Does Not See Base Stations

Have you tried a complete network power cycle? If not......

Power off the entire network......all devices.....modem, .computers, base stations, phones, printer, etc.......in any order that you wish

Wait a few minutes

Power up the modem first and let it run by itself for a minute or two

Power up the base station connected directly to the modem next, and let it run for a full minute

Power up the other base stations one at a time the same way

Keep starting up other devices one at a time about a minute apart until the entire network is back up

Check the network

If no luck, the next step would be to configure the IPv6 settings in System Preferences > Network on your Mac for a setting of Link Local Only, for both WiFi and Ethernet connections.

If still no luck, I would post in the support forum for High Sierra, since AirPort Utility is a part of the Mac operating system. The link to that forum is just below.

macOS High Sierra

Your issue is not uncommon.. I got it today; right now when checking. Even on the best optimised network I can build, every couple of weeks.. it happens. A reboot of the airport is usually more successful than rebooting the Mac itself. In my experience!!

First step I use is to refresh the network.

If you have ethernet plug directly to the Airport.. either one.. and change over Airport utility to ethernet.

For reasons unimaginable this is in the most weird place ever invented.

Also if you use different names on the two bands or are using different wifi routers to the Airport.. make a shift to local Airport.

This bought things back.

Anyway here is how to do things MANUALLY.. often helpful when stuff goes wrong.

1 Go to your wife's computer and get the IP address of the two airports.

2 In your computer go to the top menu. Configure Other.

Plug in the IP and password

Click OK.. Behold the hidden Airport appeareth!!

3 Now some fixes.

Make sure you set IP statically assuming this Airport is Bridged.

You simply change over from DHCP to Static on the Internet tab.. but you will have to fill in DNS yourself.. either same as router (gateway) or use public DNS or your ISP.. it is irrelevant and just to stop errors.

AND

Set IPv6 to Link-local only under Internet Options.

All this I did and it still fails because of issues with Bonjour. But it is occasional and a few weeks between failures and easy to recover from.

This all seems odd. Is there a way to reinstall Airport Utility?

No, the application is part of the Mac operating system and cannot be deleted or reinstalled.

On your Mac........have you performed any basic troubleshooting steps like.........

Shutting down your Mac

Waiting a minute or two

Powering the Mac back up

Checking to see if the base stations now appear

Things are obviously working on your wife's Mac, so you know that the issue is with your Mac.

Yeah.. VPN changes you to a different IP range.. so not surprising.

Thanks for reporting what might not be obvious.. still if you reboot the computer as Bob said to do it would have cut the vpn and you would be able to connect.