My home is about 25 meter long with multiple thick, brick walls in between. Signal going up is less of a problem due to wooden floors, no concrete.

Fiber optic cable enters at the front of the house. I have the mandatory router there (Zyxel). My main Deco (M9 plus) is connected by wire to the Zyxel. Main deco is in Router mode. Zyxel wifi is turned on (just found that out) and is only used by my husband if the home wifi (deco) falters.

1 room down there is my second deco (M9 plus), connected wireless. 2 rooms further, near the back door, is my third deco (M9 plus), wired directly to the main. The main reason it's there, is because that's where the cable exits the wall again.

This setup has worked well enough until recently so now I'm looking to expand my network. Weirdly enough it's often the third deco that goes offline and the backrooms and garden don't get a stable signal. Cams go offline, and so on. I'd like to buy 1 or 2 extra satelites and I'm hoping you could give me some advice on how to place them. I'd like to extend the range to the back of our garden (another 30 meters from the back door).

Some things to consider, and I think it's very relevant for our situation:

- we're a familiy of 6, with 4 teens. I work from home, the teens obviously stream and game online all the time. They're using phones and laptops, often simultaniously.

- we have a lot of smart devices, eg about 25 Hue lights, a handful of Hue switches, some Hue motion sensors, an handful of Google Nest, Google Home, Google Audio devices, a Smart TV, two Chromecasts, a couple of Sonos devices, a Hue bridge, Sonos Boost, Robot vaccuum cleaner.... even our washing machine is Smart. If I look at the connected devices at this moment, it says 41 devices.

The issue I'm having with the M9 plus right now is that it only has 2 exits, so we had to chose which devices to connect to the main and which to the satelites. So now the Sonos boost is connected to the main (and the other output is the cable that goes to the third deco). The Hue bridge is connected to the second deco. 

Like I said, it's been working fine until recently. I suspect that's because there were 3 devices added (2 laptops and a Google Home assistent).

Any suggestions on how to improve my mesh network so it'll be stable enough to deal with so many connections and is exteded for about 30 meters?