In this thread, it is mentioned that there may be issues in the R410M's firmware that cause it to hang up after a while. I upgraded the firmware according to the above and while I haven't figured out the exact time threshold, it seems to go into a sleep state of some kind after roughly 1.5 to 2 hours. I then sent it a text message and it replied about 45 minutes later, at which point all text message attempts following that moment were responded to pretty instantly. The above thread also mentions AT&T being the possible culprit.
Should executing
AT+CPSMS=0
AT+CFUN=15
resolve this issue? Think maybe what they're considering unresponsive is just an AT&T PSM network assignment?