This is the explanation for my my question about whether a time capsule can be used as a backup without being connected to a network. I had to disconnect my time capsule when I upgraded my landline to a wireless home phone in order to go to a wireless security system that would work with my 2WIRE router. The time capsule blocked the usage of my camera on my security system, not allowing the camera to connect to the 2WIRE router. Once I disconnected the time capsule, the camera was able to connect wirelessly to the 2WIRE.
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As you can read, you can do what you want. You just have to set up the Time Capsule network in AirPort Utility without connecting it to the router and ignoring the messages, and connect to the Time Capsule network. You will have to switch between the Time Capsule and the other router's networks