You might try posting this in the mandrake forum. Plus if you know what files need to get changed, then if may be that you have to do the edit's by hand in command line or log in as root for changing the files graphically.
Apart for those meagre suggestions I can't help, because that's one of the reasons that I binned my ADSL usb modem and bought an adsl modem/router. Now the device handles the connection and I can change anything I want, I just do whatever and apply the LAN settings.
Sorry I can't be of any real help.