Extending an existing wireless network using Time Capsule

I connected my Time Capsule via Ethernet cable to my network. I used the airport utility to extend my existing wireless network (Cisco router)k. After that my wireless devices were disconnected and I got a message stating "Unable to join wireless network". The macbook that I used to set up the Time Capsule showed a message along the lines that a device on the network was trying to use the macbook's IP address, I changed the macbooks IP address and then was able to connect my ipad and iphone to the wireless network but the connection was unusable as it was so slow. has anyone done this successfully and if so do you have nay advice