@Shams_USA 

The M9 is not able to take the place of a modem, but it can take the place of a router.

Since you have a modem/router, see if you are able to configure it as a bridge or in other words disable the router aspect so it just passes the internet through it directly to the M9.

So the AT&T rep was right in saying you need the modem/router, but you do not need the router aspect. If for whatever reason they tell you that you can't bridge the drive, then just disable the Wi-Fi on the modem/router, and connect your Deco to it. 

Before you connect the Deco wire a computer and test the LAN speeds to make sure that is good or else the Deco will also have the same slow speed. 

@Tony Not sure how you have yours configured but if like mine you need to go

1-from modum(power and internet connected)

2-then connect the Ethernet cord to the out port of modum

3-then connect the ethernet cord from the Modum out port to the pod in port

4-if you have a computer hard wired then connect that ethernet cord from the out port from the pod to the computer

if not then it should be connected and follow the rest of the instructions from pod to your computer

I recommned updating the ethernet cord to a thicker stronger better one from the thin flexable one that comes with it as it might be to short an

not hold up with to much movement or heat possibly,