I have the deco M5 connected to the AT&T fiber modem 5268ac.  Ethernet speeds are very slow.  When connected directly to the 5268ac gateway the speed test averages between 860 mbps-1.2 GPS.  When connected to the main deco the speeds will not exceed 300 mbps.  Some things i have tried:  Connect deco to different port on gateway...put deco QOS in custom and manually set speeds to 1000. Restart both gateway and deco.  Even the 5ghz speeds are very low compared to the gateway.  I have the deco connected to a netgear ethernet switch, but even when I connect the pc directly to the deco the speeds will not exceed 300.  The same pc gets the speeds listed above 860-1.2 gbps when connected directly to gateway.  I am using cat 6 quality cables...even tried different Ethernet cables but still the slow speed.  I have the gateway in DMZPplus mode with both wifi bands turned off.