@KiwiCharlie 

 

Deco E4 units have 100Mbps Ethernet ports - that becomes limiting factor when Internet link speed is more than 100Mbps.

 

Yes, you can add X20 as Main Deco to existent E4 Deco mesh. Follow Case 2 from this FAQ: How to replace the main Deco with another Deco

 

Adding X20 as Main Deco will improve WiFi speeds, but you will only see 750Mbps when device is connected to Main Deco. On Satellite Deco E4, WiFi speeds will not exceed 300-400Mbps.

Satellite Deco E4 that use Ethernet backhaul will still not go above 100Mbps WiFi speed.

Thank you - 300-400Mbps would be much better than the 80-100 Mbps I currently get. What settings would ensure the satellites get to the approx 300Mbps?

  @KiwiCharlie 

 

There are no settings in Deco app to control Satellite Deco speed. Satellite E4 is capable of about 300-400Mbps when it is connected wirelessly to Main Deco, based on its hardware and WiFi protocol it supports.

 

You'll get these speeds if Satellite Deco is close to Main Deco. Check Satellite Deco page in Deco app, if Signal is Strong you should expect faster WiFi speeds, if Signal is Weak don't expect much - move Satellite closer to Main Deco. 

In most cases Signal will be Medium or Fair, and that should at least break the 100Mbps WiFi speed limit you have now.