The router address you have entered is not compatible with your WAN IP address.

Unfortunately, there is not enough information here to be able to provide any answers.

What is the make and model number of the device that you call your modem?

We assume that an Ethernet cable connects from your modem to the WAN "O" port on the Time Capsule, correct?

What operating system are you using on your Mac or PC?

Just as general guideline.

The router cannot use the same IP address range (subnet) on WAN as well as LAN. If you try to do that the router has no way to route packets since everything is in the same subnet.

eg.. if you use double NAT.. and the router WAN gets an IP of 10.0.x.x (this is very common when you connect to wireless or satellite internet) you should set the LAN IP to 192.168.1.1 via the DHCP configuration. Using the standard default Airport IP range of 10.0.1.x will give you problems.

I have also had the problem happen in less exact cases when trying to use static IP address ranges.

As Bob commented to help more we need more explicit details.