Wifi only works if connected also by LAN

[mordulac]

Hi guys! I have been reading the forum, different topics, but I couldn't find a problem like mine. Long story short:

- I bought a EDIMAN EW-7811UN USB wifi adapter.
- Connected to the RPI2 and configured by ssh (detect the WLAN and successfully connect to it)
- When I reboot the PI, it is not working. If a PING the IP address (fixed IP address) I don't get any response.
- I reboot several times, also tried to config it again with the same result (I can connect to the VLAN on the xbian-config but then it is not working).
- The only that I can make it to work is connecting the RPI2 by LAN and Wifi. This way the RPI2 has to IP addresses: LAN and wifi.

It's so weird, I don't know why is happening this. Any idea would be very much appreciated.