@SimonM "Test Internet Speed" feature checks the wired speed between Main Deco and the Internet. If speed drops to 80-90Mbps, it could mean the link between Main Deco and modem/router it is connected...

@SimonM How do you measure download speed?

@stellaren How to find the hardware version on a TP-Link device

@Heymarcoh Well, then what's left is crazy creative stuff. Promote that strange Satellite Deco to Main Deco and see what happens: Changing the Main Unit for your Mesh Network

@Heymarcoh This is unusual case, what I can suggest is basic troubleshooting steps. 1. Do you have anything connected by Ethernet to that E4, such as PC, Smart TV, etc.? If yes, disconnect them and...

@stellaren If all your Deco M4 are with hardware version V4, there is beta firmware which will let you configure how Deco should connect to each other:...