No matter what I tried/settings changed etc I could not get the mac to communicate with the router with my previous setup, until I tried manually entering the static IP Address. I believe the newer modem was assigning my PowerBook an Ethernet address that was out of range and that it didn't understand.

Steps required for your modem:

1. Log into the admin interface
2. Set a static IP address for your machine (every modem is different so you will need to Google how to do thins.

Steps required for the PowerBook:

1. Open TCP/IP Control panel
2. Select the device you are connecting to the internet with via the 'Connect via' Menu (Ethernet).
3. Under configure select 'Using DHCP Server'
4. Under 'DHCP Client ID' type the static IP Address you created under the router admin page/utility
5. Enter the router address under 'Search Domains' e.g. 192.168.1.1
6. Click the 'File' menu, 'Configurations', and ensure the current profile is 'active'
7. Open the AppleTalk control panel and ensure the 'Connect via' option is the same as selected in the TCP/IP control panel.
8. Ensure AppleTalk is 'active'
9. Open a web browser to initiate the connection to the router (as Mac OS Classic does not do this automatically). If there is still no connection restart your machine and try the internet browser again.
10. In the TCP/IP control panel click the 'Edit' menu, then 'User Mode' and select 'Advanced'. Then click the new 'Options' button that appears on the bottom right of the main window. Uncheck 'Load only when needed'. This ensures that your machine maintains the internet connection until the Ethernet plug is disconnected (as Mac OS disconnects whenever there is no activity).