MTN passing the buck

I have a LG G4 smartphone which i took in for repairs at the MTN repair center in Midrand. This phone has the boot loop problem whereby the LG logo comes on upon switch on and then doesn't complete the startup process and boots again. This is a known issue that LG acknowledged in 2015/2016. My phone is still under warranty yet the repair center came back saying the phone is "uneconomical to repair" and they do not have spare parts for the LG G4. They also said they will not replace the phone !! When i complained they referred me to LG in Edenvale whom told me that since Feb of this year their policy changed which means only the Service provider, in my case MTN can carry out repairs / honor the warranty and referred me back to MTN. They also said the Repair center knows this and should not have passed the buck as they are responsible. So if MTN and LG refuse to assist me any further...what do i do???