That's the way most extenders work. It uses the last three bytes of the device's MAC address and the first 3 bytes of the extender's MAC address. However, on my extender, the first bytes is changed...

I had the same problem as you. After many, many hours of fustration, I finally saw something on my many Google searches about channels, so I changed the channel on my router from 12 to 11. It works...