@Ahmed92 

Since you are using a wired backhaul I would test to see where the slowdown may be. 

Power down all the satellite nodes, and leave only the main Deco powered on. Connect to the main Deco with an Ethernet cable and test the speed it provides. If that speed is slow, replace the Ethernet cable from the main Deco to the modem, and test again. If still slow, go into the Deco app settings, and into the QoS menu, make sure the speeds set there reflect your internet plan and are not set to be lower.

If the main Deco works fine, add a second unit, and then make a connection to the second unit. I would not expect the furthest Deco to give 500 Mbps, data needs to traverse through the nodes so that could be what you see? The Deco app will show what node you are connected to.

With the entire Deco network up you can again connect your computer via Ethernet to see if there is a specific node(s) causing it or see where it starts to drop.