Please, need help with my Nighthawk R700P, Orbi AX400 Mesh Wi-fi, and AC1900 Wi-fi Extender setup

 My devices are:   AT&T Modem (ISP provider)        ---2.4 GHz WIFI & 5 GHz WIFI operation both disabled. (someone said you can disable the router mode of the AT&T modem and just let it handle o...