@Donkeycrusher,

Are you able to tell if the devices were disconnected from the network when it occurred?

I would recommend going through the process of completely removing your Kasa app from your device and reinstalling it.

To accomplish this, go into your phone's storage settings and remove the documents and data related to the Kasa Application. Then reinstall the app from the app store.

At what point in the installation does the setup fail? Can you attach a screenshot of the message? It may simply be that your phone is not connecting back to the switch's wifi to complete the setup. You can try to remove the private addressing setting in your phone's wifi settings, and also ensure that there is no VPN enabled on your phone.

Have you tried setting up the devices with another phone or tablet?