What should a person have done when his city differed in determining Eid al-Fitr, so he completed 30 days of fasting to conform with other cities and the nation's capital?

If the people of your town adopt the sighting of the crescent moon as a legitimate sighting for the beginning of Shawwal, then it is obligatory upon you to break your fast with them. Other countries that are in agreement with your country regarding the moon's rising should also break their fast based on their sighting. However, if they rely on astronomical calculation, then it is obligatory to follow the nearest country where the crescent moon has been legitimately sighted.