You say "regular serial cable"...but have you checked the pinout? The SG series doesn't use the rollover pinout...it uses a straight-through, pins 2, 3, and 5 at a minimum. For CTS/RTS ...

You can’t reprogram the micro remotely through the ESP8266 because those ... over WiFi as if it were connected by a serial cable: the ESP8266 becomes a totally transparent WiFi-serial bridge.