Adding a net connection into a LAN

OK, this is a tricky question, despite the seemingly obvious solution.

I have a router that acts also as a cable modem - it is a Junxion box, and you plug into it a PC Card which can access Verizon's cell network to get high speed internet for locations that can't use it otherwise. The cards are meant for laptops, obviously, but this product lets you use it for more than one PC.

Problem is, I have an existing LAN used for file sharing basically, and I am trying to plug this box into the network to give everyone (or just some people if possible) access to the net over the cell network.

The box itself has two network ports and a wifi link. Currently, I have it hooked up directly to one computer and the other port goes to the LAN. That one computer gets on the net just fine. I've set the IP address of the unit so that it is within the range of the LAN. The other computers on the network can ping the router as well as being able to open its web based management page. Thus, everyone can see the router device and the router device can connect to the net.

I have set up dozens of networks before, cable and DSL, and it is simple - connect the modem to the router and viola! Internet works for all PCs that use the router. But this guy is different, clearly.

I tried plugging the IP of the router device into the proxy settings for IE, but that did not work. I don't know where else to go from there, so I thought I'd ask around.

One solution is to use the wifi of the router device, but my client would rather not install a wifi card into each PC that they want to network. They also want the benefit of the full range (distance) of the LAN and not be limited only to the range of the wireless device.

We have an NT server and we are on a domain.

Any suggestions?

Thanks!