TLDR: My two Kasa outdoor smart plugs worked beautifully with a Netgear Nighthawk x6 router. Then I upgraded to a mesh network using three TP Link Deco M9s... everything worked fine for a while but after a power outage the Kasa plugs won't connect to wifi at all. All my other IOT devices still work just fine after the outage. Fully ironic given TP Link owns Kasa.

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Long version:

I have a fair amount of IOT devices in my two story, 2000 sq ft. house, including 16 Sonos speakers, a Nest doorbell, four Nest Outdoor IQ cameras, two Google Homes (with display), five Nest smoke detectors, a few TECKIN indoor smart plugs, twelve Philips Hue bulbs, a Roomba, ADT Command Panel, and a couple of Chromecasts.

I also have the Kasa outdoor smart plugs in question.

It was previously all connected through a Netgear Nighthawk x6 router with ATT Fiber, which hardwired gets 820 mb/s download speed. It all worked pretty well together, but there was a lot of congestion with the Nighthawk as the sole access point and it was affecting playback on my Sonos speakers and I had to set the bandwidth on my Nest cameras to mid or low to avoid issues.

I did a bunch of research on mesh wifi systems to alleviate the congestion issues and settled on the TP Link M9 Plus system with three units (one hardwired and two wifi). Set it up with the same SSID and PW as the previous Nighthawk so I didn't have to go around resetting everything. 

Everything worked great for a while, INCLUDING the Kasa outdoor plugs (KP400). With the M9 system there were significantly less issues with Sonos, all Nest cameras set to high bandwith and working great, much faster wifi speeds upstairs on the opposite side of the house from the downstairs Nighthawk. I'm very happy with the M9 mesh system, and get over 300 mb/s over wifi all over the house.

I hid my SSID through the Deco app for a while for a bit more security. Everything still worked fine with the SSID hidden, including the two Kasa plugs.

Then one fateful day I had a power outage. After the power came back up, all devices reconnected just fine aside from the Kasa plugs and the Nest Cameras. For the Nest Cameras to get back online, I had to unhide the SSID and factory reset a couple of the cameras (strangely not all of them). It was a pain resetting the Nest cameras, but still they worked after resetting.

But alas the Kasa plugs would not reconnect even after unhiding the SSID and factory resetting them. I've tried unplugging the cameras for 30 seconds, factory resetting the plugs, removing them from the Kasa app, disabling the 5 GHz band on the mesh wifi (since Kasa plugs only work on 2.4 GHz), setting up the plugs inside three feet from the hardwired M9 unit, resetting the routers, resetting both the plugs and the routers at the same time, spent over an hour chatting with tech support chat and tinkering with various settings in the Deco app, and NOTHING WORKS.

The Kasa app will connect to the smart plug, allow me to input the SSID and PW, say "connecting to wifi" with this endless circle going around forever. On the plug itself, it will blink green for a while after the app says "connecting to wifi" but then go back to blinking orange and green as it does on setup. The app meanwhile still has that endless circle until I force close it.

Suspecting that maybe, just mayyyyyybe, the plugs were fried somehow in the power outage, I took the plugs to my girlfriend's house, where I gave her the old Netgear Nighthawk and it still has my same old SSID and PW. Tried setting up the plugs there (with the same SSID and PW as my current house!) and it worked FLAWLESSLY. Set up was quick and easy, it worked the first time, I could turn off and on the plugs with the app, everything. The plug got the latest firmware update after setting it up.

I then took the plugs back to my house now that they were setup and should connect automatically to the wifi since we have the same SSID and PW at her house and mine, but it still won't work! Just blinks red and says it's not connected to wifi. I factory reset it again, and get the same old endless circle on setup.

Now I could just give her the plugs too and try to find a different product to automate my outdoor plugs. That would probably be a better use of my time than endless troubleshooting and writing tomes on tp-link support forums, but I just can't. let. it. go.

It's an issue with the M9 mesh system interacting with the Kasa plug, but I have no idea what else it could be. The mesh network works just fine with ALL other devices, and the plug works just fine with another router. They just don't work together. TP Link owns Kasa, which makes it even more ridiculous - shouldn't your products work best together?

Any ideas on what else to try? (p.s. thanks for reading all this)