...and as egorgry asked, what hardware are you using to set the network connection up? The issue may not be needing to re-set everything from sratch, but you have not used the settings to make it happen at boot time. Is you system set up use DHCP? Does the ISP provider require that the IP is are dynamically asked for on boot, or is this a static setup?
If you don't know, then most likely you did not tell Mandrake (in the configure my computer/network setting) to use DHCP at boot. This is how the script knows that it is OK to automatically get the IP settings and all from the ISP server when the system boots. If the current USB modem settings seem to work, set it up again and tell the utility to allow dhcp to get the settings at boot. It is a button that must be checked, as the menu guides you through the setup of the network. When it asks for any other special settings, just hit enter and allow the utility to proceed without entering any special values.