Product: Vehicle loan or lease

On XX/XX/21 I sent a payment to Kia Motor Finance in the amount of {$550.00} using bill payer from my credit union because the Kia website where I had made my first payment had been down for several days and the payment was already due. 
On XX/XX/21 I called Kia Motor Finance because I saw on my account that the payment I had made on XX/XX/21 had not been applied to my loan and my account was showing past due. When I checked my account with my credit union I verified that the payment was paid and accepted by Kia. At that time I did see that I had inadvertently put Kias phone number XXXX in the account number section. So I called Kia to try and rectify the error. After over an hour on hold and speaking to several people I was told by XXXX that my Credit Union needed to fax them proof of payment and then my payment could be applied to my loan because she could not find the payment. This process would take approx 3-5 business days once proof of payment was received. Any other payments When Kia received the payment with an invalid/wrong account number the payment should have been rejected and returned to my credit union, but it was accepted and not credited to any account. 
On XX/XX/21 my credit union XXXX XXXX XXXX faxed the requested proof of payment to Kia Motor Finance verifying the payment and their acceptance of it. 
On XX/XX/21 made XXXX payment in the amount of {$500.00}. 
On XX/XX/21 Kia had still not credited my loan for the payment of {$550.00} made on XX/XX/21 and is still showing my account as past due. So I sent a letter to Kia Motor Finance detailing the situation and asking that the payment be applied to my loan or returned to my credit union and I also attached a copy of the proof of payment that my credit union had faxed to them. 
On XX/XX/21 my account still not credited the payment I called to see why and what the problem is. I was told I need to make the past due payment. I explained that there is no past due payment if they apply my XXXX payment. Lady verified that they had received the proof of payment from my credit union on XX/XX/21 and it is attached to my account, but that the department that may be able to help me were gone for the day. 
On XX/XX/21 received Account Past Due Notification dated XX/XX/21 stating that a payment of {$510.00} was past due, late fees added, and further collection efforts will be taken if not paid. 
On XX/XX/21 called again to see why after 3 weeks after Kia received proof of payment from my credit union they still have not applied the payment to my loan. Was told I needed to pay the past due balance of {$510.00} right now to prevent further collection and negative credit reporting. Asked to speak to a manager or supervisor who can tell me why my {$550.00} payment made XX/XX/21 still had not been applied to my loan and if it had been there would be no balance due! Spoke with XXXX from the customer assistance team. She confirmed that the proof of payment was received from my credit union along with the letter I sent. She stated that it is in verification process. I asked her how long that takes and she said 3-5 business days from when the proof of payment was received. I asked her how much longer this was going to take since it has already been 15 business days. She stated she has no idea when they will verify the payment. She stated she would escalate. 
At this time I have spent hours on the phone with Kia Motor Finance and have provided them the proof they needed to credit the {$550.00} payment to my loan. My due diligence has still not resolved the problem. My credit score is now in jeopardy and I am in the process of starting a refi on my home which this will negatively affect.