You can use the service port, but make sure you configure it correctly. Here is from a Cisco doc:

By default, the physical service port interface has a DHCP client installed and looks for an address via DHCP. The WLC attempts to request a DHCP address for the service port. If no DHCP server is available, then a DHCP request for the service port fails. Therefore, this generates the error messages.

The workaround is to configure a static IP address to the service port (even if the service port is disconnected) or have a DHCP server available to assign an IP address to the service port. Then, reload the controller, if needed.

The service port is actually reserved for out-of-band management of the controller and system recovery, and maintenance in the event of a network failure. It is also the only port that is active when the controller is in boot mode. The service port cannot carry 802.1Q tags. Therefore, it must be connected to an access port on the neighbor switch. Use of the service port is optional.

The service port interface controls communications through and is statically mapped by the system to the service port. It must have an IP address on a different subnet from the management, AP-manager, and any dynamic interfaces. Also, it cannot be mapped to a backup port. The service port can use DHCP in order to obtain an IP address, or it can be assigned a static IP address, but a default gateway cannot be assigned to the service port interface. Static routes can be defined through the controller for remote network access to the service port.

Hope this helps.