INVESTMENT ACCOUNT DEBITED

I have an account with Nedbank and investment accounts , so early this year i took a loan with them so i skipped a payment so instead of them debiting my transactional account they went ahead and debited my investment account , i tried to log a complain with their complaints departments the query is being handled by Dignity Disego she then rep**** to me stating that yes they had the right to debit the amount from the investment account and that my query would be closed.

i called their call Centre again this morning and spoke Khadia who gave a ref number *** i asked her if an investment account can ever be debited she confirmed that according to her knowledge that is not done , i want NEDBANK to please refund my money and make proper arrangements with me regarding the missed payment THEY HAVE NO RIGHT TO DEBIT MY INVESTMENT ACCOUNT THAT WAS NOT THE MANDATE I DID NOT AGREE TO IT , worst service ever and I've trying to get hold of Dignity for two days to try and understand what's going on