I wanted to do this yesterday and I have made it work as follows:

You need the smartphone that can provide a WiFi hotspot (At the end of the process this will be your "normal" smartphone).

For set-up only you need a second smartphone or tablet that can log-in to the hotspot provided by the main smartphone (it doesn't need to have it's own 4G/5G internet access, in fact it doesn't even need a SIM installed).

1/ Enable WiFi hotspot on main smartphone (Dev1).

2/ Log-in to this WiFi hotspot on second device (Dev2).

3/ If not already done so install Kasa app on Dev2 and create and/or log-in to your Kasa account.

4/ Add your Kasa device (I was using a KP105 smart plug) and when asked to select the WiFi network, choose the Dev1 hotspot connection.

5/ The Kasa device installs ok.

6/ You don't need Dev2 from now on.

7/ If not already done so install Kasa app on Dev1 and log-in to the same Kasa account as used above on Dev2.

8/ As long as the hotspot is enabled on Dev1, the Kasa device can be accessed and controlled as normal from Dev1.

9/ Program whatever timer schedules etc you want for the Kasa device.

10/ Obviously you won't be able to control the Kasa device and it won't be able to react to other Kasa device events once you are out of range or the hotspot is switched OFF because it won't have a WiFi connection .

11/ However, any timer schedules etc that you program into the Kasa device will still continue to work without the WiFi connection.

This worked for me using a Pixel5 as Dev1 and an old, unused Nexus4 as Dev2.

Hope this helps.

JR.