Miele Customer Service Rejects My Installment Request

I purchased a laundry dryer from Miele Ataşehir and processed the payment via mail order on 26.12.2023, specifying that it should be in 5 installments. The store representative, Ms. G**** Ç****, assured me that 5 installments had been set up, so I didn't feel the need to double-check. However, upon reviewing my payment on the 1st of the month, the scheduled cutoff date, I discovered that only 2 installments had been charged instead of the agreed-upon 5. I contacted the store, and they explained that due to a public holiday, the transaction would be processed on 02.01.2024, as the accounts department had sent an email on 01.01.2024. Despite this assurance, no refund was made by the 5th. While they claimed a refund was initiated, the bank requested a refund reference code, which the company failed to provide. To avoid any harm to the company, I took the initiative and repaid the difference from my own card on 03.02.2024, securing the 5 installments this time. When I contacted customer service, Ms. S**** kept me on hold for 9 minutes, pretending not to hear me. What should be my next course of action?