Misleading info.

I'm on pay as you go with Cell C. I purchased voice bundles R50 for 90min around the 30th July on recurring of which I had used around 30 minutes by the end of August.

I received a sms instructing me to purchase more airtime for the recurring to happen again which I did and had a balance of around R65. When the recurring happened, I recieved the correct amount but lost the R30 that was remaining from the previous month.

I contacted the customer care and I was advised to rather manually purchase a day before the 30 days are over.

Please can someone explain what recurring means and how it works cause I'm a little confused.