Try resetting the printer's network parameters, as follows:
1. Turn off the printer
2. Hold the "GO" and "Cancel" buttons at the same time.
3. Press and release the power button while still holding the "Go" and "Cancel" buttons.
4. Release the "Go" and "Cancel" buttons after about 6 seconds after you press the power button.
That will reset the network card to factory defaults, and you should be able to change the settings again.