@kallekoo 

Plc (powerline adapters) that you tried, what is their brand name and model number?

I had successfully connected two Deco with powerline adapters.

  @Alexandre. 

TP-LINK TL-PA7017P KIT

I've learned sense that the powerline adapters should have G.hn support, that seems to be the standard that the PX50 uses, but I still don't know if mesh will work properly with an standalone powerline solutin.

  @kallekoo 

Deco mesh will not join standalone powerline solution, but TP-Link powerline adapters may be able to work separately from PX50 and provide wired (Ethernet type) link to garage.

This is how it is supposed to work:

1. In the house, connect TL-PA7017P by Ethernet cable to available Ethernet port of Main Deco or any Satellite Deco;

2. Verify, you have Internet link from the garage, through second TL-PA7017P which you placed there. You can test that link with laptop, for example. Proceed with next step only if you have confirmed Internet link;

3. In garage, connect Satellite Deco by Ethernet cable to TL-PA7017P. See if this works: two Deco linked through pair of TL-PA7017P should establish connection with each other. If they do, in garage Satellite Deco will report Ethernet link to the rest of Deco mesh.

For this test you can borrow one of Satellite Deco from the house and bring it to garage.

If that setup works, I suggest you buy Deco X50-Outdoor for garage and use it there. If garage is climate controlled, just "regular" Deco X50 or X55 can do.

If you can't have it working in three steps I listed above, with only 3-rd step failing, it could be because TL-PA7017P conflicts with PX50 powerline. 

  @Alexandre. 

Hello.

That's just how I had them before, I have the P50 outdoor unit installed outside the garage, facing the house, and from this P50 outdoor unit i have poe going to an injector, and the injector lan side connects to this plc adapter. The other one is in my livingroom, connected to the main deco.

The backhaul (??) connection is really unstable, the garage deco keeps getting offline, and therefore causing a lot of problems in the home automation system, when the shellys toggle being available and unavailable. So it makes the home automation system totally unreliable, useless.

This problem is not of an major issue, I can live without the automations, but it seems strange that I can't get wifi to the garage, even if the distance is just about 20 meters.

The P50 outdoor unit doesn't seem to have that much of a signal strength, I have strong signal just outside of the house, and the P50 is at max 15meters away, beeing an outdoor unit, I would have expected a strong signal for this distance.

As it is now, I have a px50 unit in the window of my house, with direct sight to the P50 unit, but no luck, therefore I have to find a working solution like plc.

But the outdoor unit would benefit a lot from the built in plc, in my opinion.
Then you could place it easyer and not have to worry if the indoor wifi signal is strong enough, here in the north we have thick walls so it blocks the indoor wifi signals very good.

But maybe in the next version they will add plc supposrt for the P50 outdoor.

I will try this one more time before returning the P50, it would be nice to get it working.