CellMapper support Dual SIM Phones recording most of Dual SIM Phones with API 4.2+. API 4.2+ support by default Dual SIM mode. On CellMapper App Dual SIM settings is for old Cell ID API.
Please check Prefered SIM is First or Second SIM (Example below). Don't make mixed combinations because phones may report incorrect data.
During active calls, some phones may report incorrect data.