The E20 ethernet reader is designed on DSMR 5. When using it on a DSMR 5 supported meter, it will be powered (parasitair) via the P1 port of the Smart Meter, no extra power supply needed.
The ethernet reader is also compatible with older Smart Meters, having DSMR version 2, 3 or 4. As older versions of DSMR can’t provide enough power through the P1 port, an external power source is required to work. That can be any adapter with an USB-C port and is capable of delivering 250mA. This should be no problem for any USB-C Phone charger.