I'm posting here because I spent far too long trying to figure this one out, plus there seems to be a lack of support articles for the new KP105 Slim Smart Plug. Hopefully this will help people out.
Upon setting up the smart plugs it would always get stuck after I select my WIFI name and password, it would never connect. I tried many times, tweaks various settings on my router and nothing worked.
Turned out after finding this post: https://community.tp-link.com/en/home/forum/topic/199794 it was due to the security settings on my WIFI network.
You need to change the WIFI security settings on your 2.4GHz band from WPA2/WPA Mixed Personal to WPA2 Personal.
Stragely I have three HS100 plugs and they all worked fine, I even reset one to see if it would connect and it did, it was on the newest firmware too.
Anyway I hope this saves someone some time.