FNB credit card NOT interest free for 55 days as advertised

Interest was charged on my credit card after 30 days (on 5 Feb 2018), and not 55 days as advertised. I phoned the credit card division, which was absolutely clueless and tried to mansplain the life out of me without actually listening to my query. Furthermore, the amount of interest that was charged is far too much (amounts to an interest rate of about 47% if calculated on the full amount outstanding, for 30 days). It is my understanding that interest only STARTS accruing after 55 days???

After 15 unsuccessful minutes on the phone with the first consultant I asked to be transferred to a manager. After holding the line for AGES, I finally reached said manager, who immediately put the phone down in my ear! Worst client service experience I have ever had!