Bad service

On The 3rd of April we scheduled a flight with mango airlines from jhb to dur. Our departure time was 17:50. We arrived at the airport at 17:15 and the check in agent said the gate was closed. He was very rude and stubborn. We had to reschedule our flight which costed us 2 times the price of our original ticket. This is ridiculous and I'm a frequent flyer. Altogether we paid close to R3000. Never again with mango I hope my complaint is taken into consideration and action is taken as soon as possible