With “802.3af/at”, PoE the powering device tries to determine if the device plugged into it needs power or not, and this usually works OK.
But with passive PoE, the power is just there (often controlled by managing the device) and can damage things not expecting power to be there. So the Ethernet port of the Raspberry PI 3B+ and 4B not intended to be PoE 15V powered terminate the connection.
So you must force the ethernet interface to 100Mbit in full duplex and with “autoneg” to off. This allows you to force the configuration of the RaspberryPi ethernet interface without dialog with the source interface.
You can do this with :
ethtool -s eth0 speed 100 duplex full autoneg off
Now the Eth0 interface is UP and Power supply the “Sixfab Power Management & UPS HAT” by the J4 power input connector with a PoE 15v-12w injector.
And everything seems to be working well.
To be continued …