Regarding SMS commands, make sure you use correct syntax. Depending on the configuration of the SMS rules, you might need to add admin password before the command: <admin_passwd> <command>.
Considering that the devices are disconnected from the network, they may not have the correct time data to reboot properly.
You can also configure a separate reboot for the device modem, without restarting the device completely, in case of disconnect. In many situations, it is enough to restore data connectivity. The configuration is found in Services -> Auto reboot -> Ping/Wget reboot.
The troubleshoot files would be helpful, however, keep in mind that the logs storage is finite and, in order to get the most out of them, disconnect should have happened recently.