@GoKiwi 

If you check Deco E4 specs, you will find it has 100Mbps Ethernet ports. This is what limits Internet connection speed via the Deco E4.

If you can return that set, send it back and replace with Deco M4 set. M4 is similarly looking Deco model with similar specs compared to E4, but it has gigabit Ethernet ports and that will unlock your Internet speed.

Another option is upgrade. Get single Deco M5 or Deco M4, add it to your Deco mesh. Promote it to Main Deco: Changing the Main Unit for your Mesh Network

Use your Deco E4 as Satellites. This will also unlock your Internet speed, because your Main Deco now has gigabit Ethernet connection to router.

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On a side note: if you have Deco mesh at your house, it would make sense to disable WiFi on router.