According to the help file it should be in Advanced Wireless.
The options on this page should be changed by advanced users or if you are instructed to by one of our support personnel, as they can negatively affect the performance of your router if configured improperly.
Transmission (TX) Rates - Select the basic transfer rates based on the speed of wireless adapters on the WLAN (wireless local area network).
But it's missing from my router menu.
Firmware Version : 4.11 , Mon 02 Mar 2009