I've struggled with the same this morning, and finally managed to enter DYI mode following these steps:
Start here with 3.5
On the Sonoff device press button for 5 seconds (light pattern changes)
Press button again for 5secs (light pattern changes again)
Connect to the isteadxxxx AP using pwd 12345678
Browse to http://10.10.7.1
Enter your SSID ...