Setting an ATT PACE 5268 ac gateway in passthrough mode for a private router

I have an ATT gateway, the connection is terrible, poor range and slow speeds when multiple devices are used. I connected a new router behind it with much better results but I think I'm having double NAT issues. I tried different suggestions from forums to set the gateway in passthrough mode w/o success (DMZ+, purchasing/renting static IPs from ATT) and the ATT connected tech service. I basically need a way that my personal router works with the equivalent of a passthrough mode from the gateway.