DIAGNOSIS WiFi signals degrade severely through walls, especially brick, stucco, or metal. Your mesh system's wireless hop from router to point to backyard is too weak.
FIX Reposition your Point. It needs a strong signal from the router *and* to cover the backyard. Place it in a window or on an exterior wall that's centrally located between the router and the backyard, not tucked in a far interior room.
IF THAT DIDN'T WORK The only reliable fix for outdoor coverage is to hardwire a Point to create a strong base station. Run an Ethernet cable (outdoor-rated if necessary) from your router to a Point placed in a garage or on an exterior wall facing the yard.
HEADS UP Adding more wireless Points won't solve this if the first hop is weak. Each wireless hop cuts bandwidth significantly. Prioritize the quality of the connection between the router and the first Point.