To upgrade firmware, it says you must use the MTH network, not the home network.
Although that's what the manual states, you do not need to be on the WIU's MTH network to access LuCI. You just need to know the WIU's IP Address.
The address supplied in the instructions are what you'd use if the WIU is in MTH mode, since that's its default IP Address. I signed static IP Addresses to al 3 of my WIUs before updating them. Now, to update their firmware, I just power them on and browse to their assigned addresses.