@Kevin_Z Thank you for helping. See my reply below.

@MikeP Thank you for the suggestion. After much investigation, all devices came back online after I forced the router to rescan Wifi channels. It was odd, because no other devices in the house were...

We have a number of smart plugs and bulbs in the house. We use the Kasa app. Everything has worked faultlessly for a couple of years. Suddenly, all control has stopped working, for all devices,...