Disconnected readers will continue to function in an offline state so long as they are powered up. Their function will be based upon the information in the last compile that they completed while connected to the server. Host commands (admit, unlock, lockdown), custom rules and compiles cannot be performed when a reader is not connected to the server; nor can credentials be enrolled by presentation at that device. If new users / credentials were created after that last compile, the reader would not have learned that new information.
Device connectivity is something that needs to be addressed on the local network side. Here is a link to the section of the Pure Access manual that addresses network connectivity:
https://manual.pureaccess.com/pure-access-manual/1/en/topic/network-configuration
However, as a first troubleshooting step, you can try power-cycling the device to see if it reconnects to the server after rebooting. If the device is powered via PoE, you can power-cycle at the network switch or unplug/re-plug the device's ethernet cable at either the switch or the device itself.
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