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You probably just use the first four number buttons. Press the No 1 button 10 times till it gets to 0 and so on.
The only other Jeep Radio used the preset button to choose the location and then the tuning up and down button to select the number. You could try that way instead.
These two ways are of course for units that are standard fitment and not aftermarket units.
Take a picture of the audio unit and what year is the vehicle?
And what year and model is the vehicle?
Having spoken to a friend who is a Jeep Technician he has stated that the unit codes are either selected by pressing the preset button x many times until you get to the number required or select the position using the number and then the tune button to select the number required.
If this fails then the unit may be faulty and as such requires a new unit.