Hi,
Could you please send couple SMS'es to your device and then go to System > Administration > Troubleshoot and download the troubleshoot file?
I will report this issue to RnD and see if there's anything obvious going on with the device.
As for your request for the recommendation - we have RUTX12 that has two active modems and can do all sorts of functions like laod balancing between them, traffic splitting, using different modems and their functions without any switching time.
EB.