Cyberpine, I know this is an old post, but have you solved this?
I can help as I used to do a LOT of work with cable broadband, DSL, and wireless Internet at my last job. I have some questions, though:
Is this some sort of DSL?
If the modem is not doing ANY routing, and this is DSL, then you have some PPPoE client running on your PC to connect. If this is true, then whatever your PC's IP address is, will be your INTERNET address. If that's the case, then it is a simple matter of enabling RDP services on the PC (IF you have a version of Windows that supports it) OR installing an RDP server on the machine. Then, make sure Window Firewall (or any firewall you have installed) is allowing such connections.
On the other hand, if your modem has the Internet IP address, and your PC has a private IP address, believe it or not, your modem IS doing some rudimentary routing functions, along with a form of address translation. What is the model of the modem?
