What to do when ex spouse/ex married partner UNLAWFULLY claimed me as a dependent on 2019 taxes?

My ex spouse unlawfully claimed me as a dependent on their 2019 taxes even though I was not living with them or financially supported by them for all 365 days of the tax year (was only living with them for 8 out of 12 months of tax year). I know this because I went to file for my COVID-19 stimulus check but received the error: R0000-544-02.

How do I report their intentionally illegal activity (this was no accident)? How am I supposed to get my stimulus check when I've been ILLEGALLY claimed as a dependent on someone else's tax return?