Do the main routers provide the additional WiFi channels 12+13? (maybe JP, DE, AUS…)
The mango does not use channels 12+13 by default as they are not allowed in some countries.
If your main router has some auto channel setting or you restart it, it may change to channel 12 or 13 and Mango will not be able to reconnect.
But you can change the country settings of Mango to activate the channels: