I've noticed strange behaviour on my home network.

I am running two decos M4 connected together through ethernet via TP-Link TL-SG1008P unmanaged switch.

When I'm connected to my main network through WiFi and perform internet speed test, I get instant max speed results (250 Mbps) in the range of both main and stalite deco. When I switch to guest network and perform the same test, I'm also getting instant max speed result when I'm in the range of main deco, however, when I move to the range of satelite deco, the results are only up to 10 Mbps.

Main deco is located on the ground floor, and the satelite is placed on the first floor.

Both main and guest networks are running 2.4 GHz and 5 GHz band - sticking to one band per network doesn't change anything. I can leave 2.4 GHz on main network and keep 5GHz on the guest one, and still get the same results.

Any advice will be very appreciated.