My usage limit cannot be adjusted. Nobody can help.

Hi. I'm sad to have to log this here. I've been an MTN contract customer for about 15 years. For the first time ever I overshot my contract allowance on data, as well as the usage limit (which I didn't know I had). My iPhone basically stopped working when not on wifi. My usage limit is R250. When I follow the self service USSD path and try to change it, I am only allowed to select a value between 0 and R250. Unhelpful. I've spoken to 5 agents: they've removed limits, reset a network, and I've rebooted and removed the sim several times without success. Everybody says it looks fine, but it doesn't work. All agents have confirmed that I should be able to increase up to about R8000. Most recently (a week ago) the agent sent a request to Credit Control on my behalf? This morning my phone was dead again and the old unhelpful options remained. No progress. Please can somebody contact me and assure me that this is being handled appropriately.