@Bigredm20 

If your powerline adapters are compatible with Deco Ethernet backhaul, you can use them to link Deco in the summerhouse to Deco in the house and that will bring Deco mesh with powerline adapters speeds to the summerhouse.

You can use any Deco for that, for example Deco M5.

If your powerline adapters are from TP-Link brand, they are almost 100% guaranteed to be compatible.

You can also test that with P9, taking into consideration that for P9 Ethernet backhaul takes preference over powerline/WiFi. That will not be an elegant setup, but if it works - problem solved.

So, this is how you need to set everything:

1. In the house, connect powerline adapter to Main or Satellite Deco with Ethernet cable;

2. Place second powerline adapter in summerhouse and verify it has powerline link and that link is of acceptable speed and stability;

3. In the summerhouse, connect P9 with Ethernet cable to powerline adapter;

4. Power on P9. If it can use powerline adapters, it should report Ethernet backhaul in Deco app. If it still shows powerline/wifi backhaul or won't connect at all - that most likely means, your powerline adapters are not compatible with Deco backhaul.

If above setup failed, I would suggest getting compatible powerline adapters, such as TL-PA7017P KIT or TL-PA7017 KIT, and non-powerline Deco unit such as M5, M4 or E4.