@Richxplug 

They've abandoned us. I suspect their company is in trouble. 

I think that I may have found a solution, at least this has worked for me so far. The plug goes straight into offline if it's on my 5ghz but it seems to be working on my 2.4ghz wireless. I have had to separate the two, which is a huge PITA, and it seems to be working.

I had this issue for one of my devices. Thankfully turning off the power and turning it on again at the actual socket cured it.

@dnauk 

Sadly, that has proven to not be a working solution for most people.

I have noticed that the HS100 units that I have had issues with (retired from my system completely as they moved from mostly local only to not joining the network at all, at least most of the time) are V1.0 units. The V2 to V4.1 units we own do not exhibit these problems, as of yet anyway (you can check individual hardware versions in the app. Click on the device, click the cog icon at the top right. Click device info and read the hardware version). I have weeded out all the V1's from our system and everything is now fine.

From experience, it appears common practice amongst hardware manufacturers that when they eventually realise that certain hardware versions of their equipment exhibit faults or failures over time, they bring out a newer version that may or may not overcome the hardware shortcomings of previous versions and then ignore the complaints as long as possible until their customers either go to other brands, or just buy new ones from them. Whether this is a hardware fault related to the items network stability or a software incompatibility with some networks' hardware/software from different manufacturers is unclear at this point but I suspect it was a problem specific to V1 models and maybe newer ones also though I have no person evidence of this.

In the case of Linksys's original Velop Tri-band system, where far too many people have complained about drop outs and issues that present themselves over time, that their forum technicians repeatedly tell them to delete and reinstall the app, reset their devices (ad nauseam) and turn off certain features to see it will fix the issues,which are most likely  hardware failures. Despite this, they continue to sell these devices to the general public who blindly trust that these things are going to work long term without acknowledging the issue or even mentioning whether the units on the shelf are indeed newer hardware versions than the ones that people are having issues with. In this case alone, Tp-Link are not on their own in their silence.

I suspect some of these issues can be addressed with firmware updates but that requires the willingness of these manufacturers to actually accept the faults and try and fix them. The obvious lethergy from these manufacturers is an unfortunately identifiable trait amonst most of them (in my extensive experience) and will likely continue to be from here on. My advice is to avoid companies who don't support their customers when things go wrong, and I don't mean being around to tell them to turn stuff on and off, but to actually accept their mistakes and do something about it. I don't personally wish to avoid Tp-Link entirely as they have provided me with many reliable products over the years but the HS100 has been a mixed bag and has definitely made me hesitant to purchase new devices from them or even recommend any of their devices to friends. 

@Noslenwerdna

I believe they only operate on the 2.4ghz bandwidth

We just encountered this same issue. Our HS105 used to work just fine to control our bedroom lamp. Put a new Alexa in the same room with a different account enabled and it wouldn't work anymore. Reset the entire thing (several times) and it's always "local only." I'm extremely disappointed. 

@Clyman11 

Out of interest, what hardware version is it?
 

Once I relaxed the firewall to the iPs, the units began working. I have about 20 devices, only two were loco-only. The others still have the tight firewall profile.

Do I really need to get a network packet sniffer to figure out what ips and ports these things need to talk to? Or can KASA tell us?

Not sure if this'll solve things for everyone, but what finally worked for me was a solution I found in a different thread.

1. Log-in on the computer to alexa.amazon.com

2. Disable the Kasa skill

3. Re-enable it

4. Re-enter credentials to log-in.

Now all my plugs work again! Interested to see how many people this helps.

@localonly 

Worth a go. I'd monitor it though as I found the issue comes and go's. Fingers crossed.
 

Mine all work again now since installing the Netgear Orbi RBK50. Go figure.  Probably an incompatibility issue that Tp-link haven't (bothered) figured out.