@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...

@Zaherg Yes, it appears you've got defective unit. You did all the right tests to prove that.

@Cazza1 It is valid recommendation to make M9 Plus your Main Deco. You should do that. Also, the rest of M9 Plus should become backbone of your WiFi mesh, with M5s extending WiFi from M9s to further...

@Hispanico It is possible to set static IP for that device, but not with Deco app. I run Deco mesh in Access Point mode, too. I have more than one device with static IP address. Your Deco mesh must...