Useless MTN

According to Mtn I am one month behind on payment but can't tell me why as I have NEVER missed one payment. I went to the Mtn store in town on their system I am not behind, no one can tell me why am I one month behind. I have had this contract since 2007 and never had a problem with my payments on the 25 of each month. One agent told me in 2020 they changed their system and since then a lot of people had problems.

the call agents all have a script to talk off and can not assist, the store can't find the issue you have on their system. What can I do?!