Terrible service from MTN in Sandton City

On the 9the of Feb 2022 I went to your Sandton City branch with my MTN portable router and asked them to put 20Gb of data in it costing R 599,- I explained that I only use this router for my visits to my children in Cape town twice a year as the flat we are staying in has no Wifi. The previous 2 times it worked 100%. The salesman Lesego did take my router and proceeded to add 20Gb data. I am 80 years old and don't know how to do this. He gave it back to me with the invoice nr: ***3 for R599.- I then asked to check if it works - it did not. After talking to another expert they eventually told me: Sorry, MTN has given the sim card number away to someone else. The salesman then said, don't worry, he will fix it. He put in a new sim card and surely after that it worked. The following week I went to Cape town and not even 24 hours later the Wifi did not work anymore. I then found that the salesman only loaded 2Gb. I phoned the manager Tomi ***) and she was explained the whole story. She promised to come back to me in a minute as she had to discuss it with someone else. I did try phoning her many times and she responded eventually only ones with no promise of any kind. I feel that the inefficiency of the salesman is not my fault, as he should have found out if the sim card number is still valid. I effect paid R599.- for 2Gb data! I then had to spend at MTN in Cape town another R599.- for 20Gb added data. As a pensioner I found that this is terrible service!