There are many reasons why a device may stop working in the field.
The two major ones are:
No IP detection enabled.
A device might have connectivity issues that are beyond the communication between the WR21 and the cell tower. The router thinks the connection is there and up but there is no traffic passing over the link.
You should always have some form of IP connectivity monitoring enabled to make sure end to end connectivity is available. When a failure is detected the router can restart the IP level or even reset the cellular module.
We have a full guide how to set this up here: http://ftp1.digi.com/support/documentation/AN_007_WWAN_problem_detection_and_recovery.pdf
The second issue I have seen is that whilst Digi Engineers are great coders, sometimes bugs do get through. The last two times I have seen a bug in firmware that stops cellular connections after a period of time, setting up the router reboot watchdog when no cellular connectivity is available will make sure the device comes back online.
You can do this under the section:
Configuration - Network > Interfaces > Advanced > PPP 1 > Advanced
Find the entry:
Reboot the router after "0" consecutive connection failures
Change this value to 99.
You can also set this from the CLI as "ppp 1 rebootfails 99"
DO NOT set this to anything less than 99!!!
You want it as a failsafe not something that reboots the device frequently. Realistically, if the device gets into a failed state it will only take about 10 - 15 minutes for this counter to hit and reboot the router. Do not set this counter to anything less than 99.
Try step one followed by step two.
Also, generally if the IP connection has failed, SMS is still possible. You can setup SMS control so you can simply reboot the device remotely using the "reboot" command. There is a guide here on how to set that up: