you can ping blynk-cloud.com
for that.
You should be able to Google ways to read MAC addresses from various ESP based devices.
You should be also able to isolate your routers connected DHCP devices and figure out which one is your Sonoff.
But if you can’t do that, then as already shown, you many need to load in another example sketch, like what I provided or make something up on your own… it need not be Blynk related, but then will need a WiFi library… and read the devices MAC address with the listed command WiFi.macAddress()