I recently pulled out my Xenons from back then, I’m surprised they still work and I could configure them to use the Ethernet connection.
I did store them in a drawer because I got unmotivated to continue when the shutdown of the mesh networks and Xenon support was announced.
They have eight previously unclaimed friends that I wanted to use for a personal project, is there any chance to add these to my account? The app refuses to do so (understandably since the app and xenons went EoL years ago). Probably a little late to the game 
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Unfortunately there's no way to add to a mesh network. We didn't remove the existing mesh networks so they still work, but the infrastructure to add to them no longer exists, both in the mobile app and in the back-end. Sorry!
You can still flash them by USB and run software that doesn't need cloud or networking. I have one I use for a homemade trailer light tester that only uses BLE and works offline.