Really strange. I've found which IP is linked to the mac address of the NanoPi Neo2 (btw the network component vendor is : Chiun Mai Communication Systems, Inc).
It responds to my ping but when I'm trying to connect via ssh as root it says :
ssh: connect to host 192.168.0.21 port 22: Connection refused
It seems that DHCP is enabled but not SSH.
I've used the image : Armbian_5.83_Nanopineo2_Debian_stretch_next_4.19.38.img
The ethernet port's led is on (orange).
Thanks for your help !