I have Kasa on a Samsung Galaxy S8 which is connected to 5Ghz WiFi. Is the 5Ghz WiFi band on my phone preventing the Kasa App from finding my HS100?
No, you phone is able to see both 2.4Ghz (HS100 Wi-FI broadcasts using that band) and 5Ghz even if it is connected to a network.
Do a complete factory default on the smart plug by holding the small button on top that has a gear icon for 10 seconds. Let it boot up where the status LED blinked amber to green, you should now see the Smart Plug Wi-Fi network. If not, go to the app store and make sure your Kasa app is up to date, and try again.
Thanks for your help Tony - sorted now.