Examine or exchange the copper or fiber-optic cable with a known, good cable. Rule out any bad patch panel connections or media convertors between the source and the destination. That is, things such as surveillance cameras, wireless LAN access points, and smart lighting do not need a power outlet close to them. On the other hand, fiber optic. As for modem/router burned out by PoE, unless you are doing some cable modding it shouldn't happen. Active PoE works by a handshake across the cable that occurs before power is provided by the switch / injector. These are widely used in various data networks across industries, retail chains, traffic control systems, and other diverse applications. POST failures are usually fatal.
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