@koswo 

Deco P9 is capable of using powerline, but when it can't - troubleshooting becomes a challenge. There is no user exposure to inner workings of powerline connection of P9.

You said you added these two P9s recently. Can you still return them? If you can, there is another powerline based option that does not involve P9s, which I can discuss with you.

  @Alexandre. yes, I can still return the units.  What other solution would you suggest?

  @koswo 

Get pair of standard powerline adapters like these: https://www.tp-link.com/us/home-networking/powerline/tl-pa7017-kit/

After you configure them and verified they provide access to your home network, attach M9 Plus or M5 to the powerline adapter which is in the shed. Total expense: two powerline adapters plus M5 (or M9 Plus) - which could be compatible with the price of a pair of P9s.

Also, before buying additional Deco unit, you can take one of those you already have and use it in the shed with powerline adapters. If it works, everything will work with new Deco unit you buy, and once again I strongly recommend to choose only from M5 or M9 Plus.

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This setup will be better because with standard powerline adapters you will have better exposure to info on why they can't connect to each other. There might be not just LED lights on units, but also management interface. 

There are also tons of Internet resources available for troubleshooting just powerline adapters. 

Not every powerline adapter set is compatible with Deco mesh backhaul, hence TP-Link brand is recommended. Yet, I had M5 successfully connected through ZyXEL PLA5215 set of powerline adapters.

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My guess is, your garden shed does not have Ethernet cable going to it. If, by any chance, it has coax (TV cable) coming to it from house, there is yet another option I can share and recommend.

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I strongly recommend sending back P9s instead of troubleshooting them and integrating them in your Deco mesh. Even if you manage to make it work, adding P9s with older firmware to M9 Plus set will eliminate some nice features of Deco mesh you might find useful. Such as connection preference: How to select a preferred signal source on the Deco App

Adding M5 to your M9s is safe, Deco M5 usually gets firmware updates even before M9s.

  @koswo 

An example of powerline adapters troubleshooting resource: https://www.amazon.ca/vdp/04e47249121748a081dbb46fb5a2bc2d