Advice on adding a second wifi router to my existing NVG589

Hi Guys

Its  my first post here so please be gentle!

I have the Uverse internet only package and the mandatory NVG589 mode,/router that came with my service when we signed up over 18 months ago. 

I really want to chnage this unit for a new higher performance individul modem + router system but have recently discovered that i can not do this and i have to use the AT&T provided NVG589.  I also wanto add a shared storage device onto the network which the NVG589 will not allow me to easily do

My question is

What do i need to do within the setting sof the NVG589 to make sure i get the best performance out of my new Wifi router which is a Linksys WRT1900AC?

Thanks in advance

Ben