Red Hat 8 is an old distro, and may not support your hardware, thus you are having problems. You would be well served upgrading to something newer, such as Fedora 10 (Red Hat was forked after 9 into Fedora and Red Hat Enterprise Linux). Things may start working right away if you upgrade
If you cannot upgrade at the moment, you could still try to get things working (I strongly recommend upgrading though!)
I assume your basic setup is from the wall into the modem, then from the modem via ethernet cable into your nic card, correct?
Please post the output of the command
so we can see the exact model of your card and if it is properly loaded.