I would like to know what if AP is rebooted during losing connection between cloud.
AP doesn't have any configuration and clients can not connect to AP?
Just had the same yesterday and saw the SSID´s still, without beeing able to connect, because of the not available Radius.
AP has the config local, that´s why it´s still showing them.
Much will depend on whether the previous configuration was deemed safe. If it was it will just continue functioning as it was before the reboot. If not, it will rollback to the last known safe configuration and then continue functioning.
There's help page dedicated to this topic:
Take note that for MR there's a specific section at the bottom with more details regarding to operating in a "connectivity loss" state.
On the whole, it will keep working.
If you are using splash pages (either Meraki or externally hosted), then no.
If you are using WPA2-PSK, then yes.
If you are using WPA2-Enterprise mode and an internal RADIUS server, then yes.
If you are using WPA2-Enterprise mode with an internal RADIUS server or Meraki authentication, then no.