@DEcosse I will try this tonight. Thanks for the input.

Started with a plug last night, set it to static IP. The weird thing was after I did this, everything came back on line so I stopped there. Woke up this morning and everything was disconnected again. Added the next thing to static IP and they all came back again. Going to wait and see how long it takes to drop off again. Not sure what is going on but this seems to be somewhat working. The only difference I can think of between now and when this started happening is a smart air freshener we got a few days before this became an issue. I'll update as I find more.

Edit 1: Didn't take long for everything to go offline again. I noticed all of the devices are connecting through 5Ghz, which I see on other threads can be an issue. I'm now resetting each device and putting it on the 2.4Ghz network. One thing I noticed is that when they come on-line now, they are recognized by their model number as opposed to a long string of random letters they showed up as on the 5Ghz network. Also, one of the plugs was showing up as a windows device with the name of one of my laptops. Something was definitely confused, the mystery is what caused all of this. I'm hoping this is the solution but will be somewhat time consuming so I'll wait a while and see if the devices I've made the switch on continue to work.

Ended up moving everything to the 2.4Ghz band / assigning static IP's to EVERYTHING. Did most this morning and it lasted all day. Added the last 4 switches and although they haven't turned red, I can see through the Linksys app they are randomly disconnecting. They are also spotty on whether or not Google will communicate with them. All of this since adding the last couple. Is it possible i'm approaching the max amount of WiFi Devices once you consider Google mini's, Alexa's etc? I wouldn't think I would be but i'm kinda at a loss for how to proceed. I don't know why it would have suddenly decided this was enough.

It's been a few days now and after switching everything to the 2.4Ghz band, things seem to be alright. I had one minor hiccup where they all disconnected after adding back the final two, but since then I have had no issues. One thing I noticed on the router page was that some of the switches, while this was happening, were showing up as wired not Wifi, which I found odd. I reset the router and it seemed to correct itself. No issues since.

I've added all of the devices manually to the 2.4Ghz and I don't want to tempt fate so I don't have any of the 5Ghz screen shots showing anything.

Have not added back the air freshener back so that will be the last test.

It is just a router no extender or anything. It's set to auto update frimware and I'm not sure if it has recently or not.

Linksys WRT1900ACS FW: 2.0.2.188405

There are also pages and pages of router settings, so If I have to dig in more I'll post pictures but i'm not sure which ones you would need. Hopefully the issue has resolved itself with the 2.4Ghz solution.

Well I added the Pura back and they all disconnected an hour or so after that.

Now the question is, why? I don't think it's a Kasa issue but now I don't know what it is exactly.

Google devices are fine.

Alexa devices are fine.

maybe the router overloaded?

I guess the next step is to disconnect a few switches and see if the rest stay on line.

I should probably pursue this on another forum.

@StrangeFlavor I had very similar behaviour over the last few days with my ASUS ZENWiFi AX Router 

Everything had been working seemingly normally, although I had a few Alexa alerts that one or more devices reported offline (must have been temporary as all looked fine when checked) 

But then I was having problems with a Shelly device which kept going offline; the consensus from their FB page was they do not play well with some Mesh Routers (although in my previous home with my NetGear, this was not an issue with multiple Shellys and Kasas) 

So I separated out the bands and set the 2.4 to Legacy; now that SSID and PW remained the same, however most (but not all!!) my Kasa devices went offline; I proceeded to add them back in but noticed once I got to the last few, a couple dropped off again; I could repeat this, adding those that dropped back in and another couple dropped off!!! So it definitely appeared as if a capacity issue. 

Got tired messing with it, so restored my Router back to its previous Mesh config and assigned the Shelly to connect to the Modem WiFi directly rather than my Mesh - now all works, all the Kasas seem happy and so does the Shelly. 

But the behaviour certainly sounds similar to yours