Airport Express bringing down the entire network? I got a used Airport Express (2nd gen, the one that looks like a white Apple TV) to extend the wifi range of my Airport Extreme (the tall white monolith looking thing).*
Express rear view for reference:

I ran ethernet from the Extreme to the Express (which is the Apple-recommended method) and when I plug it in to the WAN port, it brings the entire network down… both Extreme and Express are inaccessible.
When I disconnect the ethernet cable, and the Express is connecting to the Extreme via wifi, the Express works to successfully extend wifi coverage. BUT I'd rather have it hardwired together (again, Apple-recommended, and a bunch of other reasons).
What's going on here? How do I get this work?
Here's what I know:

I actually have two Expresses (only connecting one though) and both behave exactly the same way.
I know the ethernet cable and its connection to the Extreme is working, because I had another device that used that cable/port and it was working fine (Netgear Arlo Pro Hub).
From the image above, it actually looks like the WAN and LAN ports are interchangeable(?). So I've tried on both ports. No difference.
All devices have been upgraded to the latest firmware.


*Yeah, I know: this is ancient tech. But up until this point my home wifi network has been rock solid. I got the pair of used Expresses for $30 total… so why not give this a shot?
You might be having a situation in which you have a conflict because both devices are trying to act as DHCP servers, and perhaps serving addresses in the same range. 

Make sure your new Airport Express is configured in "Bridge" mode before connecting it to your network. I don't have an Airport here, and don't remember exactly what Apple calls the WiFi Router modes.  You want a mode in which the new Airport Express does NOT do DHCP or NAT routing, and just passes packets to and from your main Airport router. 

Get it configured in Airport Utility before trying to connect it to your network. You can temporarily connect it to your computer with an Ethernet cable. (Use the Airport's LAN port for this.) 

Then, you should connect an Ethernet cable from a LAN port of your main Airport to the WAN port of your new Airport Express.