So here's what I'm trying to do:
1) AirPort connects wirelessly to master router (not an apple product) and acquires IP number.
2) AirPort is connected via an ethernet connection to another computer and that computer can access the AitPort's hard drive.
I'm sorry, but we are back to square one again.
1) You can configure the AirPort to connect wirelessly to your router.....as you have already done.....by setting it up to "join" the wireless network. But, as I already tried to explain, the AirPort performs no routing functions when it does this. So, the Ethernet ports are not enabled at all. They are inactive and cannot be used to connect any Ethernet devices. As I explained above, when the AirPort is configured to "join", it simply acts as a wireless hard drive and nothing more.
2) This cannot occur if you go with the "join" option above....since the Ethernet ports are not enabled at all when the AirPort "joins" a wireless network.
The only way that you can connect the AirPort to your current router......and have the Ethernet ports enabled.....is to use an permanent Ethernet cable connection from one of the LAN ports on your router to the WAN "O" port on the AirPort.
The wireless service on the AirPort can be turned off, so your current router will provide the wireless service for your network. Ethernet devices will be able to connect to either your router....or....to the LAN ports on the Time Capsule.
If you want to try this, I will provide the steps for you. I will assume that AirPort Utility 5.6.1 will be able to configure the new AirPort this way, but I cannot assure you that it will.....because I have not tried to use AirPort Utility 5.6.1 on a new AirPort.
So, to review the process once again......
1) The AirPort needs to be reset back to factory default settings. To do that, you would hold in the reset button on the back of the AirPort for 7-8 seconds and release. Then allow a full minute for the AirPort to restart back to a slow, blinking amber light status. The AirPort is now reset and can be reconfigured again.
2) An Ethernet cable needs to be connected from a LAN <--> port on your router to the WAN "O" port on the AirPort. This connection must be maintained at all times. The Ethernet cable length can be virtually any distance that you want......up to about 300 feet or 100 meters.....so you can locate the AirPort virtually anywhere that you want.
If you are OK with both 1) and 2), then I will provide the steps for you to set up the new AirPort.