Hmm, then I would suggest setting it in the /etc/network/interfaces file. Download an SSH client like PuTTY and connect to:
Your unique SSH password would have been included on a sticker in the MangoGT box. If you need us to look it up, please send the serial number of the MangoGT to firstname.lastname@example.org. Use the following command to edit the file:
sudo nano /etc/network/interfaces
Then change this line:
iface eth0 inet dhcp
To be like this:
iface eth0 inet static
Ctrl+x, y, Enter to save. Then sudo reboot to restart the unit with the new static network settings. Gateway and DNS servers are optional if you don't need them.