When Sonoff is offline, the LEDs generally remain lit:
Red = power
Blue = WiFi
the third and last LED is off.
On the phone appears:
It is likely that the router has received an irregular signal and does not recognize the Sonoff.
Quick solution that works 95%.
Turn off the general power (router and Sonoff) for 10 seconds.
When turned back on, the router checks all connected devices and restores the connection.
