airport time capsule joining an existing network..... um.....

If you are trying to get the Time Capsule to connect to a third party WiFi network using only a WiFi connection, this is not going to work, so you can save yourself a lot of time by not trying to do this.

Even if this were possible.......(it's not)......this would be a horrible way to set up the Time Capsule on a network because backups would take at least twice as long with twice as many chances for an error to occur.

If you want to use the Time Capsule on your NBN network, it will need to connect to your main router using a permanent wired Ethernet cable connection. No exceptions.

The Time Capsule does not need to be "in charge" of the network......it will be in Bridge Mode, allowing your main router to call the shots. You can even turn off the WiFi on the Time Capsule if you don't need it, so the Time Capsule will act like a wired hard drive.

The Time Capsule can be located anywhere that the Ethernet cable can reach....up to 100 meters or about 325 feet.

I have reconfigured our network of 2 Airport Express and 1 Airport Extreme with no real troubles except for the Time Capsule.

It helps to give specific info.

I am in Australia on NBN so do follow what you are talking about.

Who is your ISP/RSP?

What type of NBN.. FTTN, FTTP, FTTC, HFC etc.

It matters because of things like vlan on wan that Apple cannot achieve.

Is the Extreme your main router? If you are using something else make and model please.

It is difficult.. to use wireless extend with multiple items.. The issue is extend is not mesh like.. it must always link to the main airport which is wired to modem or main router. If an extender is giving better signal in the location where you place the TC, it will try to extend an extender and immediately fail.

The wrong setup will also crash the network.. so when you use wired connection the TC must create a wireless network NOT EXTEND.

As Bob has noted ethernet is solution.. or, sometimes, power line adapters can be pressed into use as ethernet substitute.

Tell us models of Extreme, Express, TC.

Honestly any of them over 5 years.. the wireless could be really poor.. and it is time since apple stepped out of the router business to replace with mesh from another brand.. perhaps using the express still as they seem pretty robust.