I have the same issue. Are you outside US/Canada?

I am new Here and I am also having the same type of issue last time. Mine just appeared last week. A couple of other folks here reported the same thing. Make sure both the Dot and your eero app are running their latest updates. Also Dots that you want to use as extended cannot be part of a speaker group. I had to remove mine from my "whole house" group. 

Mine does this when the dot is connected to my Fire TV as a home theatre. May not be the case here as you never mentioned if you connected it to a Fire TV but this definitely results in the device not showing on the network because it actually connects to the Fire TV's wifi in this case.

Hello

If your Echo Dot isn't appearing in the eero app, there are a few possible causes and solutions:

Restart the Echo Dot and eero network: Restart both devices to see if the issue is resolved.

Update the software: Check for any available software updates for the Echo Dot and eero network, and update if necessary. C4Yourself Login Check the network settings: Make sure the Echo Dot is connected to the correct Wi-Fi network, and that the network settings are correct.

Disable and re-enable the skill: Disable and then re-enable the eero skill on the Alexa app to refresh the connection.

If the issue persists, you may need to contact Amazon or eero support for further assistance.