Burnt bitter Patty

So disappointed and disgusted by the service I received yesterday at Burger King Regina Mundi, I order the R180 promo for Whopper but we got burnt patty’s that was bitter to eat we had to ask for the burger to be replaced and which the manager did but she was just want to be difficult about as if she didn’t believe us that they were burnt and she replaced the burger but they still tasted the same and I told her that their grillers were not clean and she said they clean at grillers at the end of the shift , which I did not understand and to top it off when she gave us the burger she put our fries from the initial order on a serviette the fries were cold and be burger still tasted as burnt or charred. Can they please clean their grillers and when a customer is complaining can the manager take it seriously because I don’t think I would just choose to order food and just complain if there wasn’t a need or if I was happy with the service.