That would explain why. If you use broadband with an ISP that assigns IP addresses dynamically, you need DHCP. MacTCP doesn't support the Dynamic Host Configuration Protocol.
The workaround is to use a router between your Internet connection and your MacTCP using Macintosh, and to assign a static IP address to your MacTCP running Mac. Not a major obstacle, as any Macintosh that cannot run Open Transport is not well suited for Internet use anyway. And, odds are that you'd be using a more recent machine for web surfing, thus assuring that you probably have a router anyway.