Press the desired device button, then press and hold the Setup button until the LED flashes twice and enter the code. The remote flashes twice again to indicate that you entered the code properly. Press the power button to test whether the code works correctly with the device.
If the code does not work, One for All remotes have a code cycling function. Follow the steps to begin remote setup by selecting the device and holding the Setup button. Enter 991, and look for the two blinks of the LED. Slowly alternate between pressing the power button and then the Device button, so the remote cycles through its list of codes until you discover the code that functions, which may take about 100 button presses. Once you discover a working code, press the Setup button again, and this action sets the code for the device to work with the remote.
Alternatively, input 990 instead to scan for the code from the device itself. Once the remote responds to the code, input 1, 2 and 3, waiting to see how many times the remote blinks each time. The blinks count the numbers in the device's code, which then inputs as normal.