It will be, by default, trying to supply DHCP addresses to any connected equipment - which will fight your main BT router. It will also most likely be trying to use the same default IP address as the BT.
This contains some guesswork as my D-Link is slightly different, but the principle ought to be the same.
Try connecting a single 'test' computer to it. Shut down the computer, power off the D-Link, connect only that one machine to the D-Link, then power-up the D-Link, wait 2 minutes, then boot the 'test' machine.
Using a browser, go to http://192.168.0.1 & enter the name & password - default is admin / [nothing]. Change it later if it's still at default.
Then go to setup/LAN setup & disable DHCP Server. Set the Router address to something on your current BT network, which will most likely be 192.168.0.something, so 192.168.0.252 ought to be a safe bet. Remember that number, write it on a sticker underneath the D-Link, that's how you will access it in future.
Go to Setup > Internet Setup > Manual & check 'Enable Access Point Mode'
You may need to reboot the D-Link at some point in all this, it will tell you if so.
Power-off the D-Link, & shut down the test computer. Reconnect everything as before.
Power-cycle your BT router & any other computers in the house. Power up your 'test computer'.
Uncross your fingers.
- you should now be able to use the D-Link as a 'through box' (Ethernet Switch) between your BT router & any other devices.
Go back to 192.168.0.252 & change the password, if you didn't already. You might also want to configure or even switch off the D-Link's wifi at this point.
You can get the manual for the D-Link from http://www.dlink.com/-/media/Consumer_Products/DIR/DIR%20655/Manuels/DIR655B1manual070312v23pdf.pdf
Don't use any of the setup wizards - they will try to set it back up as your main router again.