Recently there have been a number of threads in the forum about this sort of problem. Perhaps App Review is more dilligently searching methods contained in attached libraries and discovering ones they don't like. First the big question - are you using any libraries that may contain methods of which you are unaware? That is, are you certain that "no third-party payment library" exists somewhere in your frameworks and the same with a method that could alter its code?
If so, one approach is to inform App Review that you do not use any third party payment systems and you do not alter any code within the app. Any belief on their part that you do is incorrect.
If, on the other hand, you do use frameworks that could contain such capabilities then perhaps you should contact the provider of those frameworks and ask them what to do.
Did you appeal?
If so, what did you say?
How many times?
How many rejections?
What did each rejection say?
Were you told to make changes to meta data and not re-submit another build?
Were you told to resubmit?
** Note these are simply questions to learn more details about what’s transpired to this point, not suggestions on what you should do next.
If you confirm that there is no such problem with the App, you can apply for an appeal, which is also the case with one of my apps. After the appeal the day before yesterday, it was successfully approved last night. Good luck!