I am using a SIM7000 (similar to the SIM808) and am sending SMSs to it. I have found that if the device is out of range and an SMS is sent to it (device still thinks it's registered to the network, but signal strength is so low that SMSs don't show up on the device), once it's back in range with good signal strength it will never get these "missed" or and future SMSs. That is not good. I'm using a Hologram global SIM but I assume this is just an issue with how cellular networks push SMSs to modems.
My current solution is to restart the SIM7000 every hour, or open a TCP connection and close it every hour. This will force a connection so that the SMSs show up on the device. I have looked at and tried lots of AT commands but haven't found a simple one that will connect briefly but also force the SMSs to show up on the device. Any advice is appreciated.