That's correct, the only difference is that the WIFI extenders are connected to the router by cable.
There in is the issue. Appears your extenders are in “router” mode instead of “bridge” mode”. As such the extenders have DHCP capabilities to assign IP addresses.
Therefore if your iPhone connects to your router your speaker may be connected to an extender and being assigned a different IP address that is not recognized by your router to which your device is connected to. Subsequently the speaker does not appear in the app.
You can test my theory by unplugging your Sonos speakers. Disconnect the Extenders from your router and power them off. Reboot your router and let it come back. Plug in your Sonos speakers and let them come back. Delete and reinstall the Sonos app. Open the Sonos app and connect to an existing system. You should be able to see all your Sonos speakers. You will also be asked to sign in again to use all Sonos features.
Let us know the results.
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