I have set up dozens of these, always follow the same procedure, and never had any issues.

Step 1:
System Tab:
Set correct timezone
Replace ntp servers with 198.58.100.237, 209.118.204.201, 199.195.193.200, 189.211.62.210
Click 'Save'

Step 2:
Network Tab:
Click 'edit' on the WAN adaptor.
Address: 192.168.1.101 (last number can be anything you want, use something different from the original .99)
Netmask: 255.255.255.0
Gateway: 192.168.1.254 (This can be different depending on router)
Broadcast: Blank
DNS: 8.8.8.8
Click 'Save'

Step 3:
Status Tab:
Miner Configuration
Enter 3 different pools
Click 'Save'

In the top right it will now say in red something like '20 unsaved changes', click that, then save and apply on next screen.

Wait for about 10 seconds after you see it saying 'applying changes to cgminer' then type the new IP you set for the machine, im my example 192.168.1.101, and you should be off and running.  Occasionally you may need to go to system tab and reboot, or power off and on.