The increase in wireless data traffic continues and is a product of several factors. First, new technologies and capabilities enable new use cases for which new products emerge. Then, with the growing user adoption over time, the data traffic is further increased. As a result, the actual growth numbers vary year over year, but the trend is sustained. People nowadays also expect ubiquitous availability of mobile connectivity and use cloud services or stream music and video. Moreover, an increasing number of machines are being connected too. While mobile network operators (MNOs) deploy outdoor cellular networks, a significant portion of the mobile data traffic is consumed or originates indoors or while traveling in trains. To this end, MNOs constantly upgrade their networks to satisfy the capacity demand. Additionally, new and wider frequency bands are made available by regulators. However, the mid-band frequencies between 3 GHz and 6 GHz poorly enter buildings, and outdoor cells on millimeter-wave (mmWave) frequencies (24 GHz to 100 GHz) are practically unusable indoors and inside modern railways with their metallic hull and coated windows acting as Faraday cages. The increasing network densification with local cell sites (also inside shielded structures) can provide a partial solution. However, the required fiber and roll-out process is very costly and takes time. Moreover, providing sufficient capacity for the increasing data traffic demand inside buildings and trains is difficult where an optical fiber backhaul is too costly or impossible. State-of-the-art solutions are analyzed for solving the indoor capacity challenge. However, we found that these rely on or impact the existing outdoor cellular network and thus cannot provide additional capacity to the network that is used indoors. As a solution, we propose the mmWave bridge, an amplify-and-forward out-of-band repeater concept. It is a radio access technology (RAT) transparent and cost-efficient method to fronthaul mobile cells. Moreover, it provides wireless data capacity inside buildings, vehicles, or concealed areas outdoors, where low signal levels drastically limit the achievable capacity or prevent communication with distant base stations. The cellular signal of a base station is fronthauled over newly available mmWave frequencies outdoor without interfering with the existing cellular network. Mid-band frequencies are used indoors to provide sufficient coverage beyond rooms and, at the same time, benefit from the outdoor-to-indoor attenuation reducing possible interference. The benefits of the mmWave bridge are described, and we discuss the corresponding challenges and solutions. Moreover, a hardware prototype was developed based on commercial off-the-shelf components. We have tested our prototype in three use cases to demonstrate the functionality and compatibility with commercial infrastructure and mobile terminals and present the measurement results. We can show that the entire capacity of a mobile cell can be fronthauled over distances in relation to the mmWave frequency propagation and signal power. Finally, the amplify-and-forward out-of-band repeater concept is generalized regarding the fronthaul carrier frequency. The use of beamforming antennas on RAT-transparent repeaters without access to in-band beam control is investigated, and a solution with a minimal impact of a few percent in throughput reduction is presented.