Bottom line, I'd like to have my time capsule on the same wireless network.
Unless you have an older "flat" or "square" version of the Time Capsule......AND.....you have access to an older 5.x version of AirPort Utility, the Time Capsule is not going to be able to connect to your ISP router using only a wireless connection.
The Time Capsule can connect using wireless and "extend" the wireless network as well.....IF.....your "main" router is another Apple AirPort router......an AirPort Express, AirPort Extreme, or another AirPort Time Capsule.
Another way to say the same thing.....Apple uses proprietary settings for their "extend" function, which is designed to only work with other Apple routers. This setting will not work with a third-party router.
Can you clarify on exactly what you want to do?
More than likely, unless we have misunderstood the information in your post, you are going to need to plan to connect the Time Capsule to your router using a permanent, wired Ethernet cable connection.