Cellular shop mtn

Saturday i went to mtn to have my phone ( huwawei p30 lite)checked because it wasnt reading my sim card , the lady said my phone was fine and i went back home because it seemed fine after she switched it off and on, but then again on monday my phone froze again n didnt read my sim card i took it back there on today with everything and they said they have to book it for inspection and at that time i thought they are going to give me another phone because im paying for this phone and i cant be paying for a phone that im not using or not working, the lady said they cant give me another phone it against policy. I am very dissapointed and angry because this is my money i am paying for a phone that im not using and to me that is *****. Please if anyone can help me please do my heart is breaking .they are sooo rude and inconsiderate of my rights as a customer, how can i pay for a phone that is not functional and they are telling me that im gonna get it on January, im running my bussiness using that phone , im studying with that phone all of my work is on that phone, this is an unfair treatment in the meantime while they are debiting their money for nothing, how am i suppoze to carry out all of the above mentioned activities.