Plenty counterfeit Cat 6 and Cat 7 cables sold. Find short Cat 5E cable that is few years old, connect M9 with it to router and see if it brings gigabit connection back.

@tonnic WiFi6 runs on 5GHz. Just like WiFi5. In Deco app, 5GHz setting controls WiFi5 and WiFi6 simultaneously.

@antonyb12000 Your Satellite Deco M5 are very well placed. They each have good WiFi backhaul link. If someone decided their location or you did it yourself: good job. Also, your Satellite Deco are...

@antonyb12000 I have Deco M5 units with my Deco mesh, I tested their speed. Under ideal conditions, Main Deco M5 should deliver up to 475Mbps. Which means, if you are getting between 300Mbps-400Mbps...

@antonyb12000 That proves the wired link between Main Deco and modem/router is good. If there are issues with Ethernet cable between them, Main Deco may drop link speed to 100Mbps. With Deco M5 you...

@antonyb12000 You should expect betters speeds. There may be multiple reasons why you see 80Mbps. Let's start with simple one: in Deco app, there should be Test Internet Speed button on main screen....