@netrate,

When you switched networks, did you use the same SSID that was used on the old network? If so you may have needed to perform either a soft reset(wifi settings) or a hard reset(factory settings). This would remove any old network information that may have been stored on the plug.

Here is a Knowledge Base Article for the Different Types of Plug Resets: Resetting a Plug - Soft and Factory Resets

The combo device that you got is often referred to as a 'gateway' and can be used as a combination, or as only a modem unit. While this device may have provided increased speeds, many gateways do not provide the same coverage as designated routers. For this reason, many power users will exchange for a modem or place their gateway into modem-only mode and then attach a separate router. However, these Gateways are often enough for standard home users.

The most important thing to consider after getting the upgrade is how the speed and stability compared to before. If you are getting better speeds with the gateway, I would recommend disconnecting or upgrading your router. 

  @netrate,

Here is a thread that another user just posted that will be extremely helpful to you: Kasa Smart Switches: How to set up WiFi networks after changing the name or password...this is how.