I've just set up Plusnet to provide dialups for 3 networked PCs. Because they were using a hub I've simply put the 3 network cables into the router and got a broadband connection working. My problem is in getting the 3 computers to communicate with each other. Some of the original network places still connect and open but I can't connect to the computer I originally had a modem sharing set up with.
It is likely that it is something to do with subnet masks and IP addressing. If you set all three machines to obtain their IP address automatically then the router should give out the correct IP addresses and netmasks to all your machines.
Also, make sure your old connection sharing machine does not have ICS turned on still - it shouldn't matter but you may as well try it.