Can't top up my internet data for no reason...

I have been using Dstv internet for over a year & every month I can top up with 25GB should I run out of data. That was the promotion that I signed up for. Suddenly I'm being told I can't top up with my usual 25GB because I have reached limit. This month of May I have only topped up with 5GB, so where has the other 20GB gone to? The agent on the WhatsApp chat line says she can't help me, there's nothing she can do, guess not even to log a call and check why I can't be topped up with the remaining 20GB for the month. I get 25GB for day/25GB for night. But every month when my day data is depleted, the contract allows me to top up with 25GB day data. I have never not paid a single month on my invoice date, yet sudden nothing can be done. Making this whole top up process inconvenient for me as I need to do my work. This is really poor service to be denied what I signed up for with no reasonable explanation. The agent agrees that this month I have only topped up once with 5GB, yet she can't help when it comes to using the other 20GB that's left for the month. Apparently I'm left with 0GB, please make it make sense, how so ?? When is the other 20GB?? When I say I rather cancel, I'm immediately sent the link to pay cancellation fee. No alternative or retention offer to say please wait while we investigate why you have received your top up data. I see why Multichoice is losing so many customers. This is what brings companies down, poor service.