High frequency, millimeter-wave (mmWave) spectrum has emerged in the 5G/6G era as a key next-generation wireless network enabler, fulfilling user demands for high spectral efficiency and low latency wireless networks. Higher carrier frequencies offer greater network capacity: for instance, 4G LTE provides an available spectrum bandwidth of only 100 MHz, while 5G’s mmWave can easily hold spectral bandwidths more than ten times greater, enabling multi-Gbit/sec data rates. Hence, mmWave enables a plethora of applications that are currently infeasible due to their requirements of very high data rates, such as high-resolution AR/VR, video streaming, conferencing, and services in crowded urban areas. But mmWave signals are easily blocked by walls, furniture and even people and therefore have been a hurdle to achieving the technology’s full potential. We have developed and prototyped a new device, called mmWall, to help 5G signals overcome this obstacle. While 5G coverage is difficult to bring indoors as exterior building walls block mmWave signals, mmWall installed in a window can transmissively relay outdoor signals indoor and steer signals to reach all corners of a large room. Also, mmWall can reflectively steer mmWave beams at non-specular angles (those for which the angle of reflection is not equal to the angle of incidence), providing an alternative signal path when signals are blocked between an indoor Wi-Fi router and user (indoor-to-indoor reflection) or between a base station and outdoor user (outdoor-to-outdoor reflection).