Hey. The AT&T router you are getting supports 1000Mbps. So its not mandatory to get a 3rd party one. After you install the services download the Smart Home Manager app and do a speed test to check what is the real speed the router has, most probably will be around 995Mbps
*I am an AT&T employee, and the postings on this site are my own and don't necessarily represent AT&T's position, strategies or opinions.