Connecting second EAX20 using Guest SSID of first EAX20

I have two EAX20 extenders. The first extender is connected to an Access Point with SSIDs MainAP5 and MainAP24. It also has a Guest Network name of Extend5 (the 2.4GHz Guest is not configured).

I tried to configure the second extender to connect to the first extender using the Guest Extend5 SSID but get an error:

The Extender was not able to connect to your existing network.