Why Smart Plugs don't require setting port forwarding on router and how to accomplish this with NodeMCU (ESP8266)
Port forwarding is required to allow a device outside your router to access a device inside a router that performs NAT (Network Address Translation).

When a NodeMCU is acting as a simple server to respond to requests, it is inside your network, and so the port-forwarding is required so that your phone or computer on the Outside can access it.

However, for the cloud-based "smart" devices, it is different. The smart device opens a connection from itself to a server (the cloud) outside your network. In this case, the router understands that a new two-way connection is created between the cloud server and your smart device.

The only communication to your smart device is from the cloud server to the smart device. If you have an app on your mobile device or computer, that app or website only accesses the cloud server, when then passes the instruction on to your smart device. Reverse direction for status info from the smart device to your app, etc.

These smart devices rely on the infrastructure of their creators to continue to work. There are no LAN-to-LAN connections from the client to the device. Everything goes through the Internet to the remote cloud server, and back.

Many home routers support a protocol called uPNP, that allows a device inside the network to request port forwarding to itself. Some routers require this to be explicitly enabled, because it's a security risk to allow any device to be able to open holes in your firewall/router. 

To have your NodeMCU device do this, look for libraries for ESP8266 that implement uPNP. I did a quick search and found https://github.com/ofekp/TinyUPnP though I cannot vouch for it's effectiveness or safety.