Extending Wireless Network breaks entire network

This sounds as if you first configured the Express devices to "extend a wireless network" and then connected the Ethernet cable to each Express. That will create a massive feedback loop that effectively crashes the entire network.

Please power off both AirPort Express devices and disconnect the Ethernet cable from both AirPort Express devices.

Check one at a time to make sure that each Ethernet cable.....that connects back to a LAN <--> port on the Time Capsule.....is working correctly by temporarily connecting a laptop or desktop to the Ethernet cable, turning off the wireless on the computer and confirming that you can get a good Internet connection that way.

Please post back when you have completed that task and we will proceed and configure the Express devices correctly.

Thanks, I assume that you are using a Mac running a current or recent operating system. You can disconnect the Ethernet cable from the computer and turn the wireless back on at the computer at this time

Power up one Express for a few minutes

Hold in the reset button on the Express for 7 to 8 seconds and release

Allow a full minute for the Express to restart to a slow, blinking amber light status

Connect the Ethernet cable to the WAN "O" port on the Express (if the Express has 2 Ethernet ports), otherwise connect to the only Ethernet port on the Express

Click on the WiFi menu icon at the top of the Mac's screen

Locate the listing for New AirPort Base Station

Click directly on AirPort Express there

That will get the setup "wizard" running. It will take a few seconds to analyze the network and then display a screen that looks similar to what you see below, except that you will see your devices displayed and the name of your wireless network.

Type in a simple name that you want to call the AirPort Express device........Express1......for example

Click Next

Watch the next screen very carefully to confirm that the Express is being set up to extend using Ethernet. This confirms that the setup wizard is picking up the Ethernet cable connection and will configure the Express correctly.

When you see the message of Setup Complete, click Done. That's it.

The Express is now set up correctly. Set up the other Express exactly the same way except assign a different device name, like Express2, for example.

Please post back on your results.

I cannot duplicate the error that you are seeing when I set up an AirPort Extreme to extend the network using Ethernet.....and I'm using Sierra.

If I set up and AirPort Express to extend the network and it is connected to the network using Ethernet, the setup appears exactly as you see in the screenshots that I posted earlier.

It's possible though, that you might be confused, since the setup "wizard" will indicate that the AirPort Express is being set up to extend using Ethernet during the setup process. But once the Express has been set up and is operating correctly.....and you then use AirPort Utility to go in and take a look at the settings for the Express, you will see that the Express has actually been set up to Create a wireless network that uses the same wireless network name that the main AirPort is using.

In other words, If an AirPort connects to the network using Ethernet, it will always be set up to Create a wireless network. The other available setting....Extend a wireless network.....is used only if the AirPort connects to the network using wireless.

If an Express is connected using Ethernet, and it is set up to Extend a wireless network.....there will be a huge feedback loop created on the network, effectively crashing the network.