@Vumad 

 

I run Deco mesh on mix of M9 and M5, and can answer these based on my experience.

 

1) Can you add a fourth and even a fifth Deco M9 node to an existing 3-node mesh?

 

Yes, you can. Seamlessly. Note that TP-Link does not sell single M9 units, so you will have to buy two-pack or three-pack for extension. 

Alternatively, you can use Deco M5 in places whre there will be Ethernet backhaul. They are less expensive than M9 and sold in single units, as well as 2-pack and 3-pack. At my household I have two M9s with WiFi backhaul between each other and M5 with Ethernet backhaul. I plan to add another M5 to my Deco mesh, will also have Ethernet backhaul.

 

 

2) Is there a TP-Link ethernet-only powerline extender (remote end) that can be paired to the Deco M9 root node?

 

Deco units should be able to work with regular powerline adapters. According to this forum, it is hit and miss, but I tried with pair of ZyXel PLA5215 and it worked just fine.

You said your friend has powerline adapters already - test with them first, perhaps you'll be lucky and Deco units will reconginze them.

 

3) Does an M9 node automatically decide whether to use a powerline or wi-fi backhaul or is it a configuration setting?

 

Deco M9 will treat powerline connection as Ethernet connection.  You can check Deco app to see if Deco unit uses Ethernet backhaul or WiFi. If it is WiFi, that would mean Deco unit can't recognize powerline.

 

4) Do the M9 nodes all use the same wi-fi channel or do they decide which channels to use based on wi-fi performance and the channels being used by nearby devices (eg a neighbour's wi-fi, Sonos, etc)?

 

They all use the same channel, but Deco app has "Network Optimization" feature which forces Deco mesh rescan WiFi network and choose less congested channel. At least, in theory. It works for me just fine, but I have little interference from neighbours. In a place with a lot of WiFi networks looking for better channel may not give much, because they all congested.

 

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Question: what is "the Deco M9 (not plus)?" Deco offers M9Plus, and also P9. Perhaps, you were interested in P9, it has integrated powerline. M9 does not.

Thanks for the comprehensive answer and spotting my typo - I did indeed mean P9 and M9.

@Vumad 

 

Consider Deco M5 with powerline adapters your friend already has. If he doesn't like end result, rollback will be easy because all you'll have to do is remove Deco units. 

 

Of course, you can try with P9s, but it would mean removing all other powerline adapters (to avoid interference) and spending more on hardware. 

 

Make sure you buy Deco units from the place which has easy return policy, with duration of at least 1 month. One month should be enough to deploy and evaluate Deco mesh.

 

There is no TP-Link ethernet-only powerline extender that is compatible with Deco P9. You will need to have another P9 from the other end, to establish powerline connection between them.

 

P9 node automatically decides whether to use a powerline or wi-fi backhaul, it can also use Ethernet backhaul.

 

You can add a fourth and even a fifth Deco P9 node to an existing 3-node mesh.