Hey Evan,
Well if your computer continues to get a public IP address (70.x.x.x) then let's look at a few things. As you may or may not know, you should be getting a 192.168.0.x IP address from the router (be default).
Firstly, make sure the modem is connected to the WAN port and not one of the others. The modem would pass the public IP through if is wasn't in the correct port on the router.
Secondly, I've had this issue on a couple of occasions and the fix was different for them both if I remember correctly. You can set a static IP on your NIC if you're still getting the 70.x.x.x number to: 192.168.0.90 with a default gateway of 192.168.0.1. You should be able to login to the router page when you do. If not, a reset on the router is most likely required (Hold the reset button for 10 seconds using a paper clip/pin/etc). After the reset, the router should automatically issue your computer a 192.168.0.x number. Of course, remember to remove the static IP if you wish to verify the DHCP server in the router is properly sending your computer a private IP automatically.
Best of Luck,
Russ