Netgear EX6100 AC750 slow speeds

 I have recently upgraded my internet service to the 1000mbps service from AT&T and have tested the speed (wirelessly) from my i-phone and macbook when connected to the AT&T router am getting 300-500mbps.  I've reset my EX6100 / AC750 to factory settings and reconnected it to the AT&T router.  It shows a good signal from the router to the extender but in checking the speeds when connected to the extender I am still only getting 20-25 mbps...and suggestions on how to increase speed from this extender?  Thank you in advance. David K.