So now you have a wireless router that is connected to your new modem and you have two wireless systems trying to connect to it?

If you read the details in the manual on setting up your router, you will notice that you need to have a machine directly connected to the router with a CAT5 cable in order to configure the router. You will need to connect one of your machines to the router with a cable, reboot that machine, and then follow the instructions for setting up the router.

You mean I should first connect my computer to one of the lan ports on the back. But what is a CAT5 cable? Is it the ethernet cable(i.e. the one that looks like telephone wire but is thicker?)

Yes, a CAT5 cable can also be referred to as an ethernet cable or a wider phone plug. Saying CAT5 is just one of those technically-accurate ways of saying the specific type of cable.

Just plug in the cable from one of the LAN ports on the back of the router to one of your machines, reboot the machine, and then do the configuration.