The step of increasing TX Power indicates that there is more data that you need to tell to get to the bottom of this.
First, how are you checking these speeds. What is the raw speed coming out of the modem, before the router?
What distances are you measuring at? Are you making certain that no one else is online.
Finally, as you can see...only customers visit this site and are rarely here to get/give answers. Mostly we are here just to see if a new version of firmware was released. Hopefully you asked this on the normal forum and got your answers.