I have 4 x20s. My ISP router speed is 68up and 20down (tested both lan and WiFi, same result).

My deco WiFi is super inconsistent. Just now I was getting 6 down and 1 up. I went in to the deco app and clicked optimize network. Then tested again and got 50 down and 0.7 up. Then tested a LAN connection to the deco and get 68 down and 12 up.

I frequently get less than 10 down on WiFi and find myself switching WiFi off on my mobile as it's too slow. This has been happening for months. I even did a full factory reset but it didn't fix it. So I'm wondering if there is anything I can do to improve the setup. I have 4 x20s. The main one is plugged in to router via ethernet. I have around 25 to 30 devices connected. No gaming. No big downloads. Netflix once or twice a week, but otherwise just mobiles, laptops and smart home devices. I've checked traffic logs and nothing is hogging bandwidth. I have a 8 port switch connected to the main deco which goes on to connect to desktops.

Is it better to connect the additional x20s to the main one via ethernet cables? Or is it better for them to be connected to eachother by WiFi only? 

What other factors might be at play causing my slow WiFi speeds? 

Any help gratefully received. At the moment I'm just considering selling it and trying another brand.

Thanks