Unfair payment terms following credit limit reduction

I have had an FNB credit card for a couple of years, with a limit of R49600 since April 2018.

Two weeks ago FNB took a unilateral decision to reduce this to R5000 without notice nor cause. This after I transferred R53000 into that account and made a few purchases.

As if that's not enough, because I had used R11000 before the limit change, their debt collectors are calling me daily demanding immediate payment on the amount now in excess of my new limit of R5000.

Even with a credit limit of R49600 I've never paid a monthly instalment of R4000; but now they are demanding over R6000 from me right now!!!!

Meanwhile, I still have a statement issued a month ago reflecting an outstanding balance of R9017.32 on that date with a minimum required payment of R451 by 25 Oct 2019. As per that statement, I still had R33 622 available credit extended to me.

I have spent the last 2 weeks trying to get someone to explain this madness, with no success. Their call centre is non existent and when querying through the app, you are referred back to the call centre, I was even told that my query was escalated and that the turnaround time was 48 hours - needless to say, another false commitment, just like "how can we help you"?

I eventually went into a branch, and after holding on the line for close to an hour for someone to assist me (as this query can only be dealt with by their call centre), the agent gave me yet another reference number promising feedback within another 48 hours.

Is FNB entitled to do as they please with my money and accounts simply because they are privy to activity (inflows) on my accounts?!

This unilateral decision is adversely impacting my credit record and sending the message that I'm over indebted and not disciplined in handling my liabilities....

Please tell me FNB, how can you help me?! Or perhaps I should ask, how would you like to ruin my life?!