Unfortunately, there is no way to do this remotely, as commands can only be sent to devices whenever they connect to our servers.
When a CSMP fails to connect, the most common reasons for this are one of the following:
1. there is no cellular coverage at that area for AT&T or T-Mobile
2. the devices are not powered up
3. or the SIM cards in them are not the original ones they came with
Also, sometimes a tower may temporarily blacklist a SIM if it excessively tries to connect to it without authorization (for example before activation) to prevent DDoS attacks, if this is the case, you will need to power off the devices and power them on again (ie. reboot) to clear the blacklist.