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.