Increase without notification

We have a 1 year contract with planet fitness for R399, and due to covid 19 our contract was frozen for 6 months to continue after regulation lifting was implemented. Now after the frozen period the installment was increased to R439, and due to the fact that we were not informed we only leave the amount of R399 in the account for debit order. They now charge us interest due to the fact that debit order didn't go thru due to increase for 3 months