The hub registration is a function of the cloud interface. If the hub has been disconnected from the cloud for an extended period of time, the hub will no longer show under the Registered Hubs page on my.hubitat.com. To remedy the problem, first reboot the hub from Settings. If that doesn't resolve the cloud connectivity issue, then you may want to look at your network settings to make sure that the hub isn't blocked from connecting to the internet.
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