Hello,
Thank you for the detailed description of your situation.
To start with, please take note that once the device is updated to the newer firmware version, without erasing all of its settings, some residual configuration and functionality aspects from the old firmware version still remain in the device, despite the fact, that it might be using a newer firmware version.
If you have indeed deployed your devices some time ago (and if that firmware version has an issue with Reboot Scheduler functional), and updated their firmware to a newer one by keeping all previous settings, that could explain why your devices are acting in this way.
Only a fresh firmware install (without keeping any previous settings) or device reset to factory default settings allows to clean residual configuration remains from older firmware versions. If during your test this issue was indeed resolved by restoring a device to its factory settings - this approach could be used on all other devices to resolve this issue on them as well.
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If your devices have SIM cards, which are able to establish mobile data connection without an APN, you could use the RMS (Remote Management System) platform to reach and configure devices once they are reset to factory default settings (even if devices do NOT have a Public IP address).