Is my action correct when I break my fast and pray Maghrib upon the disappearance of the sun's disk behind the mountain, relying on visual observation, and differing from the Iftar time based on the internet for the Muslim community in my city in China?

It is permissible to rely on calendars unless their inaccuracy is proven, especially for Fajr and Isha prayers. The sign of sunset for the entry of Maghrib time and breaking the fast for a fasting person is the complete disappearance of the sun's disk below the horizon, not its concealment behind an obstruction. The Companions explicitly stated this with phrases like "the sun has set" and "it has been concealed by the veil." If seeing the sunset is impossible due to mountains, one can rely on another sign mentioned by the Prophet, peace and blessings be upon him, which is the approach of darkness from the east, as in the hadith of Umar ibn Al-Khattab, may Allah be pleased with him. If that is not possible, then there is no harm in relying on calendars unless their inaccuracy is proven.