Do we pray Maghrib and break our fast at sunset, or do we wait for the city's Adhan, which is delayed by 18 minutes, given that the Adhan used to coincide with sunset previously?

You should ask the people of knowledge in your country; they are best able to understand the reality of the situation. If you are certain or highly assume that the sun has set, it is permissible for you to pray and break your fast, even if the muezzin has not called the adhan; this is because the time for Maghrib begins at sunset, and breaking the fast becomes permissible at sunset, as stated in the Hadith: "When the night approaches from this side (East) and the day vanishes from this side (West) and the sun sets, then the fasting person should break his fast." Verify that the adhan is indeed delayed by 18 minutes after sunset, and do not rely on people's talk or electronic programs.