Try rebooting your (cable?) modem. The solution all depends on your network configuration. Let's assume, for example, that your Grandstream is acting as a DHCP client to either your ISP's internet gateway or to your cable modem, and a DHCP server to devices behind it. In that case, your Belkin router should be set up as a DHCP client to the Grandstream and a DHCP server to the other devices on your LAN, including your computer.

You should be able to log into your router to see what IP address it is getting on its WAN and LAN sides. You should also be able to see whether it is enabled as a DHCP client and server. If rebooting the modem doesn't help, then report back on the IP addresses of your computer (behind the Belkin) as well as the WAN and LAN IP addresses of the Belkin.