Go back into configuration from the server console.
You arrive "select local network ethernet driver", 3 options:
Keep current settings (i.e sis900...)
1. Use e100 for chipset Intel
2. manually select driver .....
You can swop NIC here by selecting "1. Use e100 for chipset Intel"|. This will free the sis900 to be used for external network. Same routine when you arrive at Select external network ethernet driver, select "1.Use sis900..."