At first I was happy until I asked to change my direct debit date to month end as I don't get money mid month

At first I was happy until I asked to change my direct debit date to month end as I don't get money mid month. I did not get a response from them, this was in July. The direct debit rejected and the charged me for an extra month which extended the loan time frame. I sent 2 more emails after and still no response and there is no option to speak to a consultant on their customer service line. They have referred me to the collections company and affected my credit. My payment date is on the 15th monthly, on the 13 Sept 2024 I paid the full outstanding amount and they are now say I owe interest for a failed payment whereas they debited me and I made a manual payment as well. They don't respond to emails, we can't speak to A CS advisor but they keep changing interest for a payment that was made