You have representation at two levels
If you live in an incorporated city, you have a mayor and city commission (or council) for that city — plus a county mayor and a county commissioner for the Board of County Commissioners district your address falls into. Everyone in Miami-Dade has a county commissioner, whether or not they also have a city one.
These aren't interchangeable. A city commissioner's staff tracks casework inside city limits; a county commissioner's staff tracks casework across their county district, which is usually much larger and includes several cities and unincorporated pockets.
Why the right office moves faster
Elected offices field constituent casework constantly, and most keep it organized by district. A report that reaches the office that actually represents your address gets tracked as casework for that district — a generic complaint sent to the wrong desk usually just gets forwarded, if it gets a response at all.
Finding yours
CivicBestie looks up your representative for you once you confirm an address, in the Report step. If you want to check it yourself first, your city's official website and Miami-Dade's county commission site both have address look-up tools for exactly this.