@Remagio 

TP-Link powerline adapters are not compatible with Deco P9. Adapters from another brand will not be compatible, too.

There may be other solutions for expanding home network over two buildings, but if you want my help with that I will need you give me more information:

1. In your garage, do you plan to use Deco attached to Ethernet cable for in garage devices, or you plan to wire them all to networking switch, or you have other plan in mind?

2. In your house, do you run Deco mesh in its default Router mode, or in Access Point mode? Access Point mode may offer more flexibility in solutions.

3. Do you have one TP-WPA271 powerline plug, or set of two?

4. How many Deco P9 you have and do you have other Deco models in your Deco mesh (for example: M5, M4)?

  @Alexandre. 

Thanks for your interest (and lots of questions)

1. currently only 1 device planned in garage (though maybe future scope for external Wifi range extender ... one day), so planning to use an Ethernet coupler.

2. Deco is currently configured as Wi-Fi router (default) - but is Ethernet connected to wired router (which has Wi-Fi switched off to avoid interferance). Didn;t know about AP config option, so keen to explore more flexibility (as long as I get to keep the whole house mesh)

3. I actually have set of 3 PowerLine plugs, but no spare plug sockets anywhere near the router, nor spare Ethernet ports in router (I'd need to buy another switch hub), though I do have plenty of spare CAT6 cable.

4. Currently 3 x Deco P9.  have contemplated buying a 4th to plug into where cable will enter the home and use it as Powerline only (wondering if an AV1000 plug would work?)

Many thanks

  @Remagio 

I would suggest switching your Deco mesh to Access Point mode regardless of how garage connectivity will be resolved. This mode allows maximum flexibility in how devices can be wired on your home network. Also, your wired router will manage all of your network. 

See: How to set up Deco to work in Access Point mode

Also: What’s the difference between Access Point mode and Router mode on the Deco?

The rest of conversation will be based on an assumption Deco mesh is in Access Point mode, which makes any wiring of devices and links discussed below possible. As I still do not have completely clear picture of your house, electric outlets and network setup, some options I discuss may not be applicable.

1. If you could extend Ethernet cable to the wired router, it'll be the best solution. 5-port gigabit switches are very inexpensive these days, if you need to expand number of Ethernet ports on your router.

In the future, if you need WiFi in garage, you can buy single Deco M5 and attach it to Ethernet cable in garage. It'll become part of your Deco mesh and you'll have WiFi coverage in garage. Your wired device can still be wired through second Ethernet port of M5.

2. A variation of Option 1. Same plan will work, if you can extend Ethernet cable to any of Deco P9 you currently have and connect cable to P9 Ethernet port.

3. With the pair of powerline adapters you can link in house end of Ethernet cable through adapters to an Ethernet port of any P9 you have, or to wired router, and that will provide you with Ethernet to the garage.

4. Yes, you can buy additional P9 to plug into where cable will enter the home. P9 will join your Deco mesh and through its Ethernet port will provide Ethernet link to the garage. Powerline Deco (just like powerline adapters) are very sensitive to quality of electric wiring, you may wish to test this will work by borrowing one of your Satellite P9 and placing it where cable will enter the home. If that works, and Ethernet/Internet speed is sufficiently fast, you will know it makes sense to buy 4th P9.

Then, if you need WiFi in garage in the future, do it with Deco M5 on another side of Ethernet cable, in garage.

5. A variation of Option 4. If WiFi signal from Deco mesh nodes is strong where cable enters the house, you can buy non-powerline Deco unit, such as M5. In will be your new Satellite Deco, wirelessly connected to the rest of Deco mesh, and through its Ethernet port and cable to garage it'll provide Ethernet link to garage.

If you need help in figuring Deco mesh WiFi signal strength where cable will enter the home, let me know.