Payment not credited to account

On the 25th September a back up battery was installed by ADT and a cash payment of R1 800.00 was made at their office in Herm**** on the 1st October. When this amount reflected as outstanding on my statement, I e-mailed proof of payment twice and spoke to the accounts person in Herm****, who was to rectify this mistake. I have just received this month's statement and the amount is now 30 days overdue. I need my account to be credited with the amount that was paid.