@Cavaleiro 

Don't do this. A lot of very useful features you have in Deco mesh running on M9 Plus require every Deco to have them. You will lose these features, even if you add P9s as Satellites. It is because P9 firmware is way behind comparing to M9 Plus. 

There is a way to achieve what you want without losing Deco mesh features you have. Get pair of TP-Link powerline adapters and single M9 Plus or M5. M5 is an option because its firmware is on par with M9 Plus.

One powerline adapter goes to the house where you will connect it by Ethernet cable to your home network. Another powerline adapter goes to garage. After you verified they have established powerline link, connect new M9/M5 to garage powrline adapter.

TP-Link powerline adapters support Deco Ethernet backhaul. If everything goes right, you should see garade Satellite Deco with Ethernet backhaul in Deco app. The speed of link will be limited by powerline speed, of course.

  @Alexandre. 

Many thanks for your reply. I hope this thread helps many others to achieve their objectives.

What peeling tp link adaptares do you suggest or that are compatible with the mesh that Deco M9 Plus operates?

I have investigated that the power line adaptars do not work with the same mesh technology but the OneMesh technology which means that the poweline will operate apart from the Decos which Ilis not the best option for me.

Edit: oh, you are suggesting to get TP Link powerline adapters that does not have wifi features, is it?

Kind regards

João 

  @Cavaleiro 

I am suggesting to get powerline adapters that do not support WiFi, and connect M5/M9 to powerline adapter that will be in the garage. I tested this setup with my M5 and two powerline adapters I have, and it worked. I recommend that with the confidence.

These are adapters that should work for you: TP-Link AV1000 Powerline Ethernet Adapter(TL-PA7017P KIT)

They have pass-through electrical socket, which is convenient. If you don't need that, check also TP-Link TL-PA7017 KIT.

You can test this in stages. Get pair of powerline adapters from the place with return policy. Put them in house and garage and see if they can establish powerline link. If they do, take one of your Satellite M9 Plus, bring it to garage and connect by Ethernet cable to powerline adapter. Check if that M9 Plus reports Ethernet backhaul. If it does, it means everything is working and new Deco unit M9/M5 you buy for garage will work over these adapters, too.

In case powerline adapters can't establish link, that means you will not be able to use powerline to bring WiFi to garage, and you should return powerline adapters to seller.

  @Alexandre. Hello,

I will try your idea and I will come back with feedback.

My deepest thanks for your time and for sharing your ideas with me (with us).

Kind regards.