Dependents claimed fraudulently

I found out my ex wrongly claimed our children on his tax return (he filed before me and mine was rejected). He then received not only the child credit for 2020 but also the subsequent stimulus funds for them. I now learned he's also getting the child credit advance payments that are being distributed now.

 

How do I go about reporting this? I tried numerous times to contact the IRS and never can get someone on the phone. Any advice would be appreciated!