Recent advances in quantum communications have underscored the crucial role of optical coherence in developing quantum networks. This resource, which is fundamental to the phase-based architecture of the quantum internet1, has enabled the only successful demonstrations of multi-node quantum networks2-4 and substantially extended the range of quantum key distribution (QKD)5. However, the scalability of coherence-based quantum protocols remains uncertain owing to the specialized hardware required, such as ultra-stable optical cavities and cryogenic photon detectors. Here we implement the coherence-based twin-field QKD protocol over a 254-kilometre commercial telecom network spanning between Frankfurt and Kehl, Germany, achieving encryption key distribution at 110 bits per second. Our results are enabled by a scalable approach to optical coherence distribution, supported by a practical system architecture and non-cryogenic single-photon detection aided by off-band phase stabilization. Our results demonstrate repeater-like quantum communication in an operational network setting, doubling the distance for practical real-world QKD implementations without cryogenic cooling. In addition, to our knowledge, we realized one of the largest QKD networks featuring measurement-device-independent properties6. Our research aligns the requirements of coherence-based quantum communication with the capabilities of existing telecommunication infrastructure, which is likely to be useful to the future of high-performance quantum networks, including the implementation of advanced quantum communication protocols, quantum repeaters, quantum sensing networks and distributed quantum computing7.