So two walls. What construction: brick, Adobe, wood, siding, stucco, metal? Insulated? Is the interior wall paper foil or flaked? All kinds of things Dan affect signal. Classic Zwave in a typical US home usually gets about 40’ of signal, although in a cement house it may get only 15 or 20. Zwave plus is noticeably farther, but still not usually more than 60 indoors.
Zigbee is more commonly used to reach outbuildings, as it does a bit better through water, including rain and snow.