My deco X25 are not getting the full speed that we are paying for.

When I plug directly into our modem, I get 800-900 mbps but when I plug into the deco, I get half that. I'm just really confused. I'm an IT Support Technician and this is stumping me.

Ive checked the QoS, the firmware, and I'm using the ethernet cable that came with the deco. I'm not sure what's going on