I have setup my internet connection like this:

Internet ----- Modem ----- Main Deco (m5) ----- TP-Link Switch (TL-SF1008D) ----- Deco 2 (m5)

My Main Deco is located in the living room, in there I get 200 Mbps (My ISP Speed, all good in here).

If I go to my bedroom (where the Deco 2 is located with Ethernet Backhaul) I always get 100 Mbps. Checking the App, for Deco 2, it says that its source is Ethernet and not Wifi (which means that the Ethernet backhaul has been successfully configured).

All the Wifi devices in my bedroom shows 100Mbps when Speed Tested using App and Web browsers. 

I have checked my QoS and I have manually configured 1000 Mbps for download and upload speeds.

If I disconnect the ethernet cable from my Deco 2 and connect it directly to another device (AppleTV), that device always shows me 200 Mbps when Speed tested (in my bedroom).

Why am I always getting half my ISP speed in my bedroom (100Mbps) using Ethernet Backhaul, when in the living room (where the Main Deco is located) I get 200 Mbps (testing the same devices in both rooms using Wifi)?