I assume you have the WCG200 connected to the WRT300N, LAN to WAN, respectively.
[wcg200](lan)<-- wire -->(wan)[wrt300n]
If so, make sure the WCG200 and WRT300N are using DIFFERENT networks. For example, if the WCG200 is 192.168.1.x, perhaps make the WRT300N 192.168.2.x.
If you’d rather not have a second router (and thus a second network and double NAT), you should instead connect them LAN to LAN.
[wcg200](lan)<-- wire -->(lan)[wrt300n]
In this case, both routers share the SAME network. You need to assign the WRT300N a static IP in the same network as the WCG200 (one that’s not being distributed by the DHCP server on WCG200), and disable the DHCP server on the WRT300N so it doesn’t conflict w/ that same DHCP server. This is usually the preferred configuration.