Attenuation speed

Hi, i have a Tp wa5210g plugged to Tp Gregorian 24dB antenna.
I receive a signal on output ethernet by Tp Wa5210g a 6Mb/s on
speed meter by my laptop.
When i use a Linksys (dd-wrt) as Access Point that it is behind
the Cpe and Antenna i notice an attenuation from 3-4 Mb/s to less
1Mb/s on devices linked to Ap Linksys (setted default power
When i try an output power lower of Ap (1mW) i notice an
improvement from 3-4Mb/s to 1,50Mb/s but is not enough.
Do you know how can i solve?