@walkinthepark When speed drops to 90/25, open Deco app and you should see "Abnormal WAN speed" message. What you describe points at issue with Ethernet cable between Main Deco and NBN NTD. On Main...

@Fifator You can't change WiFi signal strength with Deco E4, and also you should know that Deco E4 come with 100Mbps Ethernet ports, not gigabit.

@Cazza1 Your Deco M9 Plus units have firmware 1.2 from 2019. It is so old it can't be updated through Deco app. Deco app will say "up-to-date" while in fact it should be saying "unable to update."...

@J-gh Yes, you can use second Ethernet port on Main Deco to connect a TP-Link power line adapter to use with a computer upstairs. This should work just fine. I have two recommendation for you, if you...

@sghatuwa Deco XE75 Pro two generations ahead of M5. They will work together, but can't promise it'll be smooth. Instead, I would recommend to put aside (decomission) M5 and get pair of Deco X50 or...

@madmax44 The primary router must be from the Deco family, to work together with E4 as Satellites. Considering what you have, I would recommend M4, M5, or M9Plus as your primary unit. With that...