@J.Oc 

Deco E4 has 100Mbps Ethernet ports, this will be the maximum Internet speed it delivers. Main Deco E4 connection to fibre router is where speed is limited by 100Mbps.

For your Internet speed of 500Mbps, Deco E4 is not the right choice. Return it to seller, and replace with Deco set consisting of one of the following Deco models: Deco M4, Deco S4, or Deco M5.

Those have gigabit Ethernet ports and should deliver faster Internet speeds than Deco E4.

With Deco S4/M4/M5, you should be getting Internet speed of up to 500Mbps on Main Deco and half of that on Satellite Deco. These are WiFi5 capable Deco devices. If you want even faster speeds, for example because you plan to upgrade your fibre to gigabit in the not so far future, you should consider Deco hardware that supports WiFi 6.