Our terminals are offline.
I tried factory resetting the device, but the same thing happened.
An offline response is received from the API that checks the terminal status.
If we ignore that the terminal is offline and request payment, the payment will be processed.
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Additional Documentation
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Currently, the Devices API doesn’t support real-time heartbeat checks for a device. A device has a 15 minute interval to return an up-to-date status. If the device hasn’t returned a status update after approximately 30 minutes, then the device is likely to be offline. 
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I thought it was related to network speed, so I connected two of the terminals by wire, and the rest also went online.