how can i change my internet connection from Airport Extreme to my Time Machine

My laptop is connected to my Airport Extreme on the third floor.

If i take my laptop to the first floor to my Time Machine, my connection is lost with Airport Extreme.

It looks very logical to me that my laptop will automatically be connected to my Time Machine.......

But it does not......

What am i doing wrong..........has anybody an answer for me?

Thanks in advance

Regards

Patrick