Overvoltage protection for DC controlled Device
Assuming that you only need to protect the +12V-POE output, we can ignore the LM2596 circuit.
A fuse and zener will protect against many faults, but depending on your current requirements, there may be a sweet spot where a slight overload will burn out the zener before the fuse blows.
An SCR crowbar circuit with a properly sized SCR is robust enough to handle any situation.
The zener voltages in the schematic will need to be adjusted appropriately. The default values in the simulator are not suitable.
https://www.electronics-notes.com/articles/analogue_circuits/thyristor-scr-triac/overvoltage-protection-crowbar-circuit.php



simulate this circuit &ndash; Schematic created using CircuitLab