Disappointed

I got the month to month wifi router on the 26/04/2023 and I paid an upront amount of R307 which was pro-rata for April and may premium or R250,,, but to my surprise my account was debited R539 on 05/05/2023 and all of a sudden when I call them they don't know where the R307 went to and they couldn't tell me why was I charged R539... So I was charged R846 for a R250 service plus free router how???????? Please can someone explain