Thank you for reaching out - I am happy to answer your question.
The advanced child tax credit is issued based on the prior year tax return. The IRS began sending the payments automatically to the person that claimed the eligible child on their 2020 tax return.
If your ex-spouse is claiming the child this year, you should unenroll from receiving the advanced payment - you will likely be required to repay the amount of the advanced tax credit payment received.
The IRS as a site where you can unenroll (stop) receiving the advanced payment.
https://www.irs.gov/credits-deductions/child-tax-credit-update-portal
Kind regards,