The DECT Manager is supplied via PoE as standard. So if the network is PoE-capable, it does not need a power supply unit.
Otherwise there are the following possibilities (either/or):
1- Set up PoE
2- Buy the power supply unit according to the installation instructions (chapter "Accessories").
According to my information, this should be Siemens item C39280-Z4-C706 (please check your own documents, as the N720 is not explicitly mentioned on the power supply product page). If I understand correctly, the part number of the compatible power supply should also be on the back or bottom of the N720.
Gigaset replacement power supply for DX600A/DX800A/DL500