@1hatetechn0l0gy 

Depending how far is the barn you could use an EAP225-outdoor hang on the house, in line of sight to the barn, connected with an ethernet cable to your router.

  @1hatetechn0l0gy 

You can use Deco, but when using Deco units, the main one should be connected (wired) to your router.

That means if your router is in the basement you have to use at least two - one wired to your router and X50-outdoor (wireless) hang on the house, in line of sight to the barn.

In this case Deco should be in AP mode.

If you can't route an ethernet cable from your basement, you could use a PLC (powerline adapters).

If you can route a cable to an EAP225-Outdoor that would be the best approach in terms of bandwidth and stability.

  @1hatetechn0l0gy 

Basically you have to setup your topology like in the "Deco in AP mode" guide:

Here's what would be my approach if I can't run an ethernet cables throughout the house.

Most probably the extender unit (Deco M3W) is not in the range of the main Deco E3 unit and that's why it can't be detected by the app.

Replacing this unit with Deco S4/M4 probably will solve this issue. Relocating the Deco M3W extender in the barn loft and test the connection would be my next step.

If the Deco app still can't see the Deco M3W extender (buildings are metal sided), I'll add Deco X50-Outdoor (hang on the house, in line of sight to the barn) to widen the wireless coverage toward the barn: