@Aimytran723 

What is the model of the router?

Try verifying the wireless password you set up for the smart plug is the right one for your network by forgetting and connecting back to the network on your smartphone. 

@Aimytran723 

Switch model: KP400

HW Ver: 2.1

Router: ASUS RT-AC68R

Router Firmware: 3.0.0.4.384_32799-gfe72567

Last Updated: 5/6/2020

I am having the same issue with a KP400 smart switch I purchased from Amazon. I even created a brand new guest WiFi network, and it failes to connect to that as well. No other devices in the house are having issues connecting to the router (WeMo dimmers, thermostat, garage door opener). 

If the smartphone has any VPN enabled, that needs to be turned off.

It would also be best to use a different device as well to try and set up the smart plug.

@Tony 

That was the problem. Thanks for the tip. 

@Aimytran723 

Call the support of your domain