Hi talk2sp, please can u elaborate more in your explanation for the problem. are you unable to assign IP address to your eth0? you are saying that it is connected directly to a modem. does the modem have dhcp on it configured and your eth0 is not getting the pushed IP?
or maybe your server is not seeing eth0 properly and not able to configure the hardware. I once faced this problem. the driver for my NIC was there installed by default but the NIC was not functioning until I got the newest driver from Intel and install it.
or maybe everything is setup good onto your server but the routing has a problem.
not to throw many ideas without a known issue, so please provide us with little more details, thank you