There is one specific IR codeset for your particular model, which is 1581
It is possible, however, that no matter what code is used, your remote will not be able to control the devices you wish to use it with. Cable/satellite/AV Component manufacturers include basic functionality for common devices at the time the remote is made, but are not able to provide coverage for every make and model, and are not able to update for every new device that comes out, so there is no guarantee of compatibility. This problem is compounded when using an "off brand" product like the Niko, which are rarely compatible. In short, if the 1581 code does not work, then the cox remote is simply not capable of working with this model tv.
Your best option may be to purchase a more robust, true universal remote to control all of your devices, such as a logitech harmony (click here for example) that will work for all of your devices. It is programmed through an easy to use software program on your computer, which downloads the most up to date remote and device codes from the internet, so it is virtually guaranteed to work with all equipment. It will also give you easy to use macro functionality, so you can press a single button to watch a movie for example, and it will turn on the dvd player, tv, and surround system, tune to the correct inputs on the tv, and start the movie playing, etc. Because of the online database with specific codes for each model, this type of remote has much greater functionality than the basic universal remotes (including the one that came with your system.)