Sonoff devices appear offline - repairing seems to be a only solution

I have not changed anything in my lan/wan system for over a year, some Sonoff devices are offline.
trying to cut off the power for a while then turn it on again but it does not reconnect.
When I put them on pairing mode it reconnects without a problem with the same name.
Most of the devices are inside switches and I have to take the switch out turn pairing mode and so on.
can someone help me with this case?
Do you know why those devices are going offline?
Do you know how to reconnect them without pairing them again?
Do you know how to turn Sonof mini into pairing mode without pressing a button (cutting power for a while or doing something using a switch)?

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