Does your Arduino board have a power led? Is it on during sleep?

@Efflon
Did you read the PDF file? There I explain the methodology
From an MQTT client I publish commands which the Sonoff subscribes to.
These commands allow me to add to or delete from an array of SSID/Password pairs held in a file in the ESP8266 SPIFSS file system.
On power up I scan the local networks for SSID's and search the local file for matching SSID's.
If a match is found, I use it for connecting to the local WiFI network.
Does that help?