It is not that easy. If it does work Cradlepoint will likely have a hardware solution.

The problem you will likely have is the broadband modem cards need to have the ability to allow a WAN session to be initiated from the internet. Many broadband modems do not allow this. In my market less than a year ago Verizon was the only provder that had modem that allowed this. TMobile, AT&T, Sprint modems did not.

Clarify this with the ISP and be prepared to test this. The cellular store employees will tell you it works because they don't understand specifically what you're asking.