Tenant Disputes Late Fees

I pay rent using what called "Flex" which the apartment complex offers tenants. It allows us to pay rent in * payments instead on everything on the 1st of the month. On April *, * I funded my "Flex" account in advance so the funds would be available when the app performs the "Auto-Pay" function on My 1st. On the 3rd of May I believe, I was contacted via email by a customer service representative of Flex and informed me that they were having difficulties accessing the rental portal and asked me to return to the Flex App and perform what is referred to as a "Self-Submit" option and gave me brief instructions. I returned to the Flex App and performed this "Self-Submit" function and my payment went through successfully. I took a screen shot of the rental portal receipt proving the rent for May was successfully accepted and paid and sent this image as an attachment to the Flex App Customer Service Representative. From that point on I didn't give the rental payment another thought since I received the payment success confirmation receipt of the Rental Payment Portal of the apartment complex where I live. As mentioned I never returned to look at my my emails because payment was successful. Yesterday evening I opened my email and I received a message from my apartment complex telling me that my rent is late and now instead of paying $*,* I owe over $*,*. I spoke to whom I learned is the new property manager this morning and was told that they have the right to charge me late fees but my argument is the late payment is not my fault! I’ve even shown them scree shot images of my rent portal receipt and the conversations I have had with the customer service representative from the Flex App.

I did show these screen shots of the rental payment receipt and the conversations I had with the Customer Service person this morning and the Property Manager said I still need to pay the late fees

Are these free services? I lost my job last month!

What is the contact inforamtion for these services