If shielded network cables are used, this is most likely the cause.
For PoE on SYMPL Core Sp, unshielded ethernet cables must be used!
SYMPL Core S does not support PoE, only SYMPL Core Sp.
Power over Ethernet (PoE) not working
If PoE on a SYMPL Core Sp does not work, but 24V directly from PSU works.