@icordova 

Hi, welcome to the community.

Since you have tested that the Backyard signal is weak or doesn't get a signal at all., a satellite Deco here might have an unstable connection from the main M5.

Would there be many devices that need a Wi-Fi network? and How about their distance to the main Deco?

If the main house and backyard shared the same electric meter/power grid/service panel, and there are mainly smart home devices backyard, like security cameras, you could consider adding a pair of powerline adapters, such as TL-WPA4220:

https://www.tp-link.com/en/powerline/

And you could also have a look at this link to know more about other extenders:

https://www.tp-link.com/en/blog/1241/what-is-a-wifi-booster-and-how-to-choose-the-best-one/

Thank you very much.

Best regards.