Does you're modem also have a built in DHCP router, and is the service turned on?  Some do, and some don't.  If you have an old router laying around you could try that, or pick up a used one for cheap off of fleabay.

ETA:


I just looked up the specs on your modem, it doesn't appear to include a DHCP server.  You will need a router, or a computer set up as a DHCP server to provide IP addresses to your miners.  Again, a cheap router is your best bet, and since you don't need it to provide wifi you'll be able to find one that's very affordably priced.