Want Time Capsule far away from my modem. How to add to existing Airport?

Question

Hi,
I have a dome-shaped Airport Extreme Base Station (AEBS) connected to a Comcast modem. I want to leave it as it is, but I would like my Time Capsule (TC), to be located far away from the modem, to "join" or "extend" that AEBS network.
I had it set up that way at one point, but then switched out the TC drive to a new 2 TB and then connected it directly through Ethernet to my two MacBooks to back them up more quickly. (FYI, I upgrade MB drives a lot and they aren't always recognized immediately by the TC, sometimes have to delete a sparsebundle and start a new backup--why I have to do that for it to work sometimes, no idea). But after backing up my new MB drives over Ethernet, I can't now for the life of me seem to get both the AEBS and TC onto one AEBS network. My Airport Express joins the AEBS just fine to play music, but not the TC anymore. It only gives me the "extend" option and then tells me that my AEBS "cannot be extended."
My dome-shaped AEBS lists version 5.7 and was WEP 128 secured, but I tried WPA2 Personal most recently as the TC didn't list a WEP 128 option in Airport Utility. The TC lists version 7.4.2. I have the AEBS set to b/g and the TC to its full range.
I know how to use my Mac pretty well, but I only mess with setting up Airports or TCs once a year at the most. I don't know how all of the moving parts work or work together. Various dialogue boxes will confuse the ** out of me. YET when setting up the TC originally somehow there was an option to have the TC "join" the AEBS network, not sure if since updates have removed that ability. Only since replacing the drive and turning off the TCs wireless settings to make it work over Ethernet, has the TC no longer been willing to join the AEBS.
I don't want to be a networking master, I just want to be able to configure my network quickly and easily so my less valuable (to me) AEBS will be in my living room with my modem while my entire electronic life is backed up safely in the attic to my TC that stay out of sight from thieves, etc. You'd think Apple would have built-in this ability MUCH more easily than they have! What's the point of having your only backup sitting right in the living room in the middle of all products you're backing up? If your laptops are valuable enough to steal, so is the shiny white Apple box underneath your TV. And then you have no data, no photos, no memories.
OK, done editorializing. Anyone know how to walk me through getting this BACKWARDS TC/Airport setup running? And anyone know how I could then revert to Ethernet if I want a faster connected backup sometime and then jump right back to my BACKWARDS wireless setup? Tremendously grateful for any help!

Answer 1

My Airport Express joins the AEBS just fine to play music, but not the TC anymore. It only gives me the "extend" option and then tells me that my AEBS "cannot be extended."
When configuring the TC with the AirPort Utility, you just need to hold down the Option key before making the selection in order to get access to the "Join a wireless network" choice.
My dome-shaped AEBS lists version 5.7 and was WEP 128 secured, but I tried WPA2 Personal most recently as the TC didn't list a WEP 128 option in Airport Utility. The TC lists version 7.4.2. I have the AEBS set to b/g and the TC to its full range.
First off, WEP is no longer considered secure, so it is highly recommended that you use either WPA or WPA2 for security. However, if you must use WEP, you would have to change the Radio Mode on the TC to one of the non-"n" choices in order to configure it for WEP 64- or 128-bit.
YET when setting up the TC originally somehow there was an option to have the TC "join" the AEBS network, not sure if since updates have removed that ability.
Again, holding down the Option key will provide this option. It was not removed with any of the firmware updates.