I have 2 KL130 bulbs and a strip (KL430) all on latest firmware. I've had them about a year. All connected to the same WiFi under a single group. Yesterday, as often happens, they weren't responding to the app. This is pretty common, because these aren't great quality products and they frequently disconnect. Usually I can just turn my phone WiFi on and off and the connection will restore. This time however, 2 of the 3 would not reconnect. I tried resetting one of the bulbs, but that didn't work. I can go all the way through setup until the bulb flashes red/green/blue/white, but then it reports that it's "unable to connect to home network". Obviously, there is nothing wrong with my home network, all my other devices work just fine. It gets stuck on "connecting smart bulb to your home network", where it hangs while the light cycles through colors and then fails. It disappears, then reappears, from my device group, remaining disconnected and unavailable.
I left it overnight and just used the wall switches to turn everything off (except one bulb that was still responding at the end of the night). When I woke up, even the remaining bulb stopped responding. I did a full factory reset on both bulbs multiple times. I don't want to reset my light strip because then I lose all of my customized settings and strip paintings which took a long time to design (this data should be easily archived in the app to allow you to copy configurations, but the app is extremely poorly designed). But at this point, nothing will even connect.
Went on to chat support which was pretty helpful, we discovered that there's some issue with the 2.4GHz band on my home network which just started, but strangely, my main network 2.4GHz doesn't work but if I make a 2.4GHz guest network, that works fine. So I switched my phone to the newly created 2.4GHz-only guest network and all my devices popped back up on my kasa app (with all settings reset). Further troubleshooting of my router and network is now needed but the problem with the devices was purely due to whatever connectivity issue my network is experiencing.