To answer a two year old question for those getting here from search engine results:
Reserved IP and DHCP configuration isn't accessible via the Cox Panoramic Wifi App. You'll need to navigate to the router control panel via your browser (PC preferable, but possible on mobile), which will be 192.168.0.1. From there, log in with the username and password (default: "admin" and "password", you'll be prompted to set a new one).
In the router dashboard, select the "Connected Devices" menu option and find the device you want to configure. Hit Edit, then click the option button to switch from DHCP to Reserved IP.
If your device is not already connected but needs a reserved IP, you can select the option for "Add device with reserved IP" and add its MAC address.
