I just bought the Deco M4 mesh kit with 2 nodes. The main Deco is in the living room, connected to my ISP router via Ethernet cable, and the second is in the hallway about 8m (26ft) away, with a direct line of sight from the main one (no obstacles in between), connected wirelessly. My laptop (Macbook Pro) is in the office, about 6m (20ft) from the hallway Deco, with direct line of sight towards it. Think of the layout as an "L" shape, where the main Deco is at the top of the L, the second Deco is at the knee and the laptop is at the bottom end. All devices are about 1m (3.3ft) from the ground.

My internet speed is 250 Mbps, but my laptop only gets around 15 Mbps, and very unstable. I've tried many different placements for the Decos, but this is the best I've achieved in terms of connection stability and speed.

Testing the speeds via Fast.com:

Connected to the ISP router wifi network, standing next to it: 250 Mbps
Connected to the main Deco, standing next to it: 120 Mbps

Connected to the main Deco, standing about 6m (20ft) from it: 80Mbps

Connected to the hallway Deco, standing next to it: 120 Mbps

Connected to the hallway Deco, in the office about 6m (20ft) from it: 5 Mbps

I checked Wireless Diagnostics on the laptop, and the hallway Deco's RSSI is at -51, which is a decent signal strength.

When in the office, both my phone and my laptop constantly get disconnected from the network, and speeds are very slow. My apartment has only one floor, and 120m² (1,290ft²) – it shouldn't be this complicated. Why am I not getting the full 250 Mbps when standing next to the Decos, and why is the connection and speed dropping so much in such a short distance from the second Deco? I can still pick up the main Deco signal from the office, though the strength is much weaker than the hallway Deco – could this be interfering with the connection?