How do I connect a Netgear Mesh 6 system to an ATT Uverse Pace model 5268AC modem m

running 2 routers on a network would put you in a double nat. 

Your best options are to either place the Pace in passthrough/dmz mode or to place the Orbi in AP mode. 

I will make a statement that att devices aren't the best at allowing devices to function properly in their passthrough mode. So if you go that route, try it and if things aren't functioning properly, try the ap mode on the orbi. (access point)

https://forums.att.com/conversations/att-internet-equipment/how-do-i-put-pace-5268ac-router-in-passthrough-bridge-mode/5defc347bad5f2f606eb37df

https://kb.netgear.com/31218/How-do-I-configure-my-Orbi-router-to-act-as-an-access-point