You can have multiple access points share the same SSID (and the same credentials) and smart clients will automatically select the one with the strongest signal and roam seamlessly from one to another. See the SuperUser question: How can I get the same SSID for multiple access points? for more details.
If you have connected the Airport Time Capsule to the WLAN router, you should be able to access it when connected to the router. Maybe you have to configure the router to forward traffic from WLAN to the LAN.
Especially having different SSIDs on 2.4 and 5 GHz on the same router seem to make no sense to me unless you have the special need to separate them.