So doing some research on the Internet, seems like 3G mobile modem devices are supported on some of these routers:
"One of the most unique features to this router which is a compelling selling point as well is built-in cellular modem support. For example, you can connect a USB 3G or EV-DO networking card to the router and it will provide Internet access for you network via the cellular modem in the case that you're ISP's connection fails. While probably not a practical thing for a home setting, a small business may find this feature crucial if your business has important time-sensitive data that needs to be sent out. Keep in mind that you will have to subscribe to an Internet data plan from a cellular carrier in order for this feature to work."
Didn't mention any non 3G devices and can only speculate if they would work or not.
Is the mobile modem you are asking about a 3G modem or non 3G?