Apartment is threatening to report me to credit collection

I rented an apartment with my brother in august 2022, he moved out in may 2023. My rent was $1000 monthly. June,July the apartment portal is showing me as moved out. It wouldn’t let me pay. I paid total by July. August I renewed my rent with only my name in the lease. My rent went from $1000 to $1100. I paid $1100 in august, September 2023. October 2023 they changed their portal, same issue was happening, it won’t let me access the portal. I went to the office and it was fixed, it was showing that I owed $1600, the then property manager said it’s an issue with their system and to pay it and I will get a discount the next month. I paid it expecting a discount. The following month the manager & assistant we’re no longer there. Apparently they resigned, there was a new person there and she told me she cannot help me and referred me to the head office. At the head office I was told that the cause was because my brother did not notarized the move out document in may (this is the mistake of the then manager, she made us sign it at the office but never told us anything about getting it notarized, infact they made us come to the office to fill the form) now the head office person resent me the form and had me take it to him to notarized it, I did and submitted it, the moment I submitted it the system told me I need to renew my lease ( I renewed it in august) however because it was notarized they could no longer find my lease and and I was told I have to pay month to month which was $1500 and I refused stating that it was not fair. I was going back and forth in the email, telling them I cannot pay month to month. I was given a notice to move out by Jan 4 and pay the balance of $1650 or risk been taken to the court. I paid it and moved out in December 28. A week later, they are calling me that I owed them rent!!! I already paid the stipulated amount and I don’t owe them. They are threatening to put it in my credit. Can someone advise me on what to do sincerely and this happened in houston. What can I do??? I have been trying to reach lawyers and they want to charge me $5000 it’s just not worth it. What can I do sincerely? Any agency that can help with this