What must one do if they doubt having touched their penis after ablution, and did not feel it at the time of touching, but later became certain that they had touched it?

You must strive to ensure purity from impurity. Once urination ceases, wash the organ and sprinkle water on your garment to قطع the whispers (of Shaytan). Ablution is valid as long as you are not certain of touching the penis. Do not give in to whispers, seek refuge in Allah from Satan, and spit to your left three times when you feel his whispers.