Possible to use Airport Extr and Express for WIRED Guest Network?

Question

Situation and landscape:
My house has a Comcast cable-modem, which is connected to an Airport Extreme (AEx).
The AEx is running WPA WiFi, and this is all good.
There is one ethernet wire running out of the AEx, which goes 150 feet to the rear of my house, across a short corridor (protected from weather with insulated pipe) and into my garage apartment, where it terminates into an Airport Express (APress.)   This garage apartment is frequently used as part of a rental property.  The APress is extending the same WiFi SSID/settings.
When guests arrive, they are able to plug-in their notebook computers to the APress ethernet port, or use the WiFi.
Well, the problem with that, obviously, is that they can "see" the other computers on the network, and printers, etc.
It would be perfect to set configure the AEx and APress to the WiFi "guest network."  However, there are problems with this:
1)  Believe it or not, many guests still use direct-plug and do not have nor know how to set up their WiFi -- so they MUST have a direct ethernet wire.
2)  When I use Airport Utility for the APress, it does not show any WiFi guest network option -- maybe because another dependent setting is not allowing this (i.e. Bridge Mode?")
3)  The distance between the AEx and this garage apartment is too long between them to shoot a WiFi guest network from AEx and to be picked-up by the Airport Express... and there's a center core in my old house that is impervious to all radio frequencies and could block a nuclear blast.  Well, it causes a degradation of wireless.   And this brings us back to #1 above...in that I need a cable ethernet option.
Picture attached of current landscape:
So, maybe....
I've spent the better part of a couple of hours searching here, particularly for the terms "access point" but the terminology isn't what I need.  What I wonder if perhaps I need to place an APress beside the AEx, turn on Guest Network at the AEx, then "pick it up" with a second APress, and carry the ethernet signal to the garage apartment and allow guest WiFi and wired.  (see second picture)   Will this work?

Answer 1

OK, here how to set this up.
Open up AirPort Utility 5.6.1, select the Express, and click Manual Setup
Click the Wireless tab located below the icons
If you want the Guest Network to have a different SSID (recommended), then change the name of the wireless network, adjust the security settings if needed, and change the Wireless Password and Verify
Click the Internet icon up at the top of the window
Click the Internet Connection tab just below the icons
Change the setting for Connection Sharing to Share a public IP address
Click the DHCP tab located under the icons
Change the DHCP Beginning Address to read something different.....like 10.0.5.2
Click Update and give the Express a full minute to restart
At this point, the Express indicator light will be slowly blinking amber
Open up AirPort Utility again, select the Express and click directly on the word Status (2nd line)
You should see a Double NAT notice with an option to "ignore" the item
There may also be an Setup over WAN notice with an option to "ignore" the item
Click in the boxes to ignore both items, then click Update again and the Express will restart and display a green light
Try things out to verify that the Guest Network cannot "see" any devices on the main network...and vice versa.