@FrankyNep 

One of the Deco units must be wired to a Ethernet cable, but in your case if there is not a option for a Ethernet cable between you can look at to possibilties.

1. Use two CPE access points to send the internet signal from one to another and at the end point, put a wireless router: https://www.tp-link.com/us/support/faq/1226/

2. From the house with the router, wire a EAP225-Outdoor device to broadcast the Wi-Fi signal like how a router does indoors, the EAP will do that outdoors.

If the distance is not great some users have used a range extender like the RE450 and placed that as close indoors to the backyard and the signal was able to reach where they needed.

So it might take some testing as every environment is different to find what works for you.