Thanks a lot, I purchased the exact same board from banggood, followed this guide and have mine setup, but found a few bugs in your code, and signed up just to comment

1st:
Might just be me, but when you said
"Now go to back to the IDE and click tools -> Board - > Boards Manager and search ESP"
I also needed to import one called thinger

2nd:
Where it says

#include <ThingerESP8266.h>

#define USERNAME "YourUserName"
#define DEVICE_ID "YourDeviceID"
#define DEVICE_CREDENTIAL "YourCredentials"

#define SSID "YourSSID(WIFINAME)"
#define SSID_PASSWORD "WIFIPASSWORD"
#define POWER_SWITCH D7

ThingerWifi thing (USERNAME, DEVICE_ID, DEVICE_CREDENTIAL);

We are importing ThingerESP8266.h, but at bottom we are trying to 'use' ThingerWifi so I had to change this last line to

ThingerESP8266 thing (USERNAME, DEVICE_ID, DEVICE_CREDENTIAL);

3rd:
The code
#define POWER_SWITCH D7
and instructions
"Locate the pins D3 and GND on the nodeMCU"
seem to conflict to me. They obviously need to be the same pin. I just plugged the jumper onto D7 pin

PS:
Not a bug, but I also used a small fan splitter cable to allow me to plug both this and my regular power button in.