Billing Issue

Good Day

I have had numerous problems with MTN since my upgrade on the 1st of this month. so i upgraded to 40GB / 40GB which is supposed to be R 300 on the 1st August 2019.

on the 12th only 20GB was loaded on my account. Called MTN and explained, they fixed it but then my bill went from paying around 750 for both contracts on my name to R 1 303.81 which is due on the 30th August.

i called MTN about this they gave me query numbers : ***, ***, ***. then they close the queries without explaining.

i then called MTN again, and the lady that i spoke to told me they have adjusted my premium for next month because of the error that they made on my invoice, and that next month my bill is going to come back low. I then told her, i understand that they credited my next bill, but problem is i dont have R1300 to pay mtn. and obviously if i pay the normal installment that i pay every month which 750-800 then MTN will lock my line and i will be listed as a late payer for month August? am i correct?

Can someone please help me with invoicing me right? i do not have the 1300 that you guys want from me. i dont want this to affect my credit record. Cellphone number in question : *** Regards