I have  about 10 of these plugs, and a couple of the outdoor KP400.

all have been set up using the iPhone app without an issue until yesterday.

1. The KP400 got through the setup process up to the final point where I chose my home network and password for it to connect to and it would not connect. I tried multiple times without success. I then re-tried using the app on my iPad and, voila, it worked. The Kasa settings are the same on my phone and iPad (ie local network turned on). The only change since previous trouble free setups of prior plugs is my iPhone iOS to 14.2. My iPad is on iOS 14.1.

2. Today, trying to set up two new HS 105's, I expected the same problem, but no... neither the iPad or iPhone app worked. Same issue, got all the way through setup to the final point after I chose my home network and password - would not connect. BUT if I instead tried to connect to my home extender RE230 wireless signal xxxx-EXT, it worked just fine. I have changed no router settings. 
 

so now most of my devices connect through my "regular" network, xxxxx, but two connect though xxxxx-EXT. 

has anyone else encountered this? Any explanation come to mind?