Connecting PoE and DC power supply at the same time
When two power sources are connected, the device cannot tell which power will be used.
Even if you connect the power supply first and then connect the PoE, the choice of power will alter based on the voltage of the adapter.
If the PoE hub does not enough power it may cut its power, which may cause network issues. The excessive power may damage the device caused by burning.
Furthermore if two power sources are connected, unwanted electric current may cause a noise so it is recommended that you only use one power source.