Vw SA just a ripoff

I bought a vehicle its a polo 2018 1.0 comfortline it is now on 37000 case.I took the car to vw polokwane after it was showing a check coolant light and also I realised ther was a leak somewhere because i found a small amount of water on the floor underneath the vehicle,then I took the vehicle to vw as it is still under warranty. To my surprise they couldn't find anything wrong ther ok I took the vehicle but the following 2 days I experienced again a check coolant light and I topped it up then booked the car in again. This time around they managed to see something which they said is leaking radiator ,they then ordered a radiator then called me back ones it has arrived to say I must bring the car for radiator replacement.after some hours they called me to tell me I must come to the dealership to come see something,I get ther the showing me a white substance or build-up that is in the cooling system pipes and they tell me they can't fix that under warranty because of those substance. To my frustrations I called vw SA to explain the treatment I'm subjected to because that car since I bought no-one touched it except vw technicians,now they telling me is my fault because I put in a different coolant than G13 in the vehicle,I told them I have not put in any anti freezer,That they are the ones whom should know why their coolant had done what it had done. Vw SA is just a disgrace because they don't want to own up to their ******* parts,I can't buy a brand new car and pay so much money and also be denied warranty under such a lame excuse,I will never buy a vw car ever again. Peter can u help me with this situation a device me were to got lay a complaint against VW SA.