Hi, first of all, thank you very much for helping me like that !
I'm a Noob with my sensor but i'm a network guy (it's my job !) I know how to find an ip address !
I know pretty much all my device on my network so i just look on my dhcp server to see if new device show up . Also i use a software to scan every device on the network
I don't map the mac adress because i didn't have the time. Like I said it work only one time.
I was afraid that was a physical problems so I retry with the first sketch
( https://www.mysensors.org/build/mqtt_gateway )
And it work ! I ping it without any problem...
EDIT : I just saw this in my code :
//W5100 Ethernet module SPI enable (optional if using a shield/module that manages SPI_EN signal) //#define MY_W5100_SPI_EN 4
Do I need to uncomment the second line ?