Telkom doesn't want more business

After repeated unavailability of my ADSL line I app**** for a Telkom Mobile Home Office LTE account with a 20Gb limit. The process was flawless and easy for me because Telkom outsources the sales. Now that I am connected I realised that 20Gb is way to little and I would like to upgrade to 50 Gb, in other words, give Telkom mobile more money for more capacity. After phoning I was told to go to a shop which is inconvenient for me. After going to the store I was told I can't upgrade for three months and I can only do it in store. How can this be if I originally appled over telephone and email and I pay promptly? Why can't I upgrade?