No other replies? OK not being an RH person I don't know specific details. But there should be some configuration utility to set up the modem along with a corresponding menu for connecting/disconnecting.
As for the LAN login, does the machine hang if not on its LAN or does it boot up OK? Maybe all you have to do is use some RH network config tool and remove the setting for the gateway machine, or turn off DHCP (the thing that assigns an address when the machine connects to a LAN). If it hangs, maybe the last message it puts out will give a clue as to why. Then you need to boot off a floppy or boot CD to fix the problem.
But this is a pretty old version of RH. Wouldn't you be better off wiping the old installation altogether and installing something more up to date?