The i38hg is the indoor unit for the uverse inid bonded pair. Your profile would be 25M allowing for Max internet speed of 18M. You did not mention if you also have HDTV service which could affect your speed.
When running speed tests be hardwired into the yellow port of the i38 on a Ethernet cable, no recording or watching HD shows, prefer speedtest.net run several tests and average for best.
If your tests show 15 or above you are good, wireless is not guaranteed due to many factors beyond control, a quick search should bring up many posts here is one to start with »forums.att.com/t5/Reside ··· /3261745
. Hope this sheds some light.