OK, here is what I would suggest:
Temporarily, power off the TC and move the TC close to the Asus router
Use a spare Ethernet cable.....any length will do.....and connect it from one off the LAN <--> ports on the back of the Asus router (there are usually 4 Ethernet ports, use any one that you want) to the WAN "O" port on the TC
Power up the TC and let it run for a minute or two
Using a pen or something similar, hold in the small reset button on the back of the TC for 7-8 seconds, then release
Allow a full minute for the TC to restart
Click the WiFi menu at the top of the Mac's screen
Click directly on Time Capsule just below the listing of New AirPort Base Station
When you click on Time Capsule, that will start up the AirPort Utility setup "wizard". It will take a few seconds to display a screen that looks like the example below, except that you will see your Time Capsule pictured.....not an AirPort Express as you see below.
Network Name......Type in the exact same wireless network name that the Asus is using
Base Station Name.....Type in a short simple name that you want to call the Time Capsule
Password......Type in the same password that the Asus wireless is using
Verify......Type in the same password again to confirm
The "wizard" will set up everything for you correctly and automatically. When you see the message of Setup Complete, click Done.
Now the TC has been set up correctly. When you have the long Ethernet cable installed, move the TC where it will be needed and make the connections, power up the TC, and you are all set.
Post back when you have completed the setup and installation of the TC and we'll set up the AirPort Express.
Will the AirPort Express use the same wireless network name as the Asus as well, or do you want to set up a different network where the Express will be located?