Custom flashed Sonoff basic doesn't work when connected to main power

Question

I flashed a sketch to a Sonoff basic switch R2 through Arduino IDE (with FTDI). Flash is completed and Sonoff runs the sketch successfully. The code is just a typical setup for making esp8266 a soft access point.

#include <ESP8266WiFi.h>

WiFiServer server(80);
IPAddress staticIP(192,168,1,22);
IPAddress gateway(192,168,1,9);
IPAddress subnet(255,255,255,0);

byte relayPin = 12;

void setup()
{

 pinMode(relayPin, OUTPUT);
 digitalWrite(relayPin, HIGH);
 WiFi.mode(WIFI_AP);
 WiFi.softAP("****", "****");
 WiFi.softAPConfig(staticIP, gateway, subnet);
 delay(1000);
 server.begin();


 }


void loop()
{

if(WiFi.softAPgetStationNum() > 0) {
digitalWrite(relayPin, LOW);
delay(200);
digitalWrite(relayPin, HIGH);
delay(200);
digitalWrite(relayPin, LOW);
delay(200);
digitalWrite(relayPin, HIGH);
delay(200);
digitalWrite(relayPin, LOW);

}

delay(500);
}

But when I connect Sonoff to main power it doesn't respond to the programmed sketch and it constantly just turns on/off the lamp every 500ms-1000ms approximately and when I push the Sonoff's button relay stops being heard, but the light still turns on and off.

Any insights?