With it not recognizing the modem (I assume you mean a dialup card) it means that you can't conect to the internet at all, because you have no connection to it. Do you happen to know what kind of modem it is? That might help find a way to get mandrake to recognize the hardware.
Now, if its a ethernet card, its quite possible that the actual card or the parent stuff wasn't installed in the intial kernel building, and you will have to go into your make.config file. activate the card(parent directory of cards) and recompile the kernel, this would fix it if thats a issue.
A hardware issue that there isn't software for is a bit harder to fix ^.^