A wireless broadband 3g modem is a completely different technology than WiFI. The dongle you have is designed to only talk to a cell tower and then "repeat" the signal to normal WiFI end devices. This device is different than most I have seen. It is actually a wireless hotspot that connects similar to a USB modem dongle. It still does not have the ability to connect to a WiFI modem/router as a client.
You will be better off just buying a device designed to be a wifi repeater it is much more likely to be compatible with your router.