@Winterland
In theory, any Deco model can run with any other Deco model. In practice, I would recommend staying with the hardware that supports same type of WiFi standards. Mixing Deco devices that support WiFi5 with those that support WiFi6 could trigger issues that are hard to troubleshoot.
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Which means, in your case you should be adding Deco hardware that supports WiFi5. The choice is substantial. These are your options:
Deco S4/M4 - M4 is similar to S4, almost no difference but model name. If you want to add similar Deco to your mesh, choose from S4 and M4.
Deco E4 - Similar to S4, but with 100Mbps Ethernet ports instead of gigabit. As Satellite Deco wirelessly connected to Deco mesh and if nothing is wired to it, E4 is as good as S4. Deco E4 often sells for less than S4/M4, but if you use it the right way, it is as good as S4 and saves you money.
Deco M5 - more compact form factor than S4/M4, blends nicely in the environment. The benefit of M5 is firmware 1.6 which will let you configure Deco nodes to connect in daisy chain. For example, if you want to have this:
Main Deco<-WiFi backhaul->Satellite Deco1<-WiFi backhaul->Satellite Deco2
you should be buying M5 for Satellite Deco 2. In case you are fine with all Satellite Deco connecting to Main Deco, M5 is just one of many options.
Deco M9 Plus - the most powerful and most expensive (per unit) of all of them. If your Main Deco runs hot on CPU utilization, it would make sense to replace Main S4 with Main M9 Plus. Otherwise, Deco M9 Plus shines only when it runs in pairs, because with its tri-band hardware Satellite M9 connected to Main M9 will deliver faster speeds than any other Deco from that list.
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Exotic ones: Deco P9, they can use powerline for backhaul. Unless you really, really need that, don't buy them.
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Before you buy more Deco, check if your Satellite Deco are properly placed. Open their page in Deco app and check what it says under Signal Source. Example:
For the best Satellite Deco performance, there should be at least two blue bars. If there is less, move Satellite Deco closer to Main Deco, that should improve its performance.
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I mentioned Main Deco CPU utilization. To check Main Deco utilization, login to Main Deco web interface using its IP address, should be http://192.168.68.1 if you run Deco mesh in its default Router mode. Scroll down to the bottom of the web page and you should see it reporting CPU Load and Memory Usage, in real time. Example:
Wait 10-15 seconds till it settles down, because usually right after login these numbers spike high. When numbers got down and are steady, CPU utilization should be is under 60-70% and Memory usage under 60-70%. If it gets close to these values or above them constantly, consider buying M9 Plus for your Main Deco.