The parent who claims the child as a dependent on their 2021 tax return is the parent who will get the enhanced child tax credit. If that is not you, but you take the advance credit payment, you will have to pay it back.
If you are planning to give your ex a form 8332 to allow them to claim the child in 2021 (per court order or agreement) then you should opt out (unenroll) from the advance payments.
https://www.irs.gov/credits-deductions/advance-child-tax-credit-payments-in-2021
Separately, if the child lives in your home more than half the year, you will still be entitled to claim head of household status, the dependent care credit, and EIC (if you are eligible) — those three tax benefits can't be waived or transferred to the non-custodial parent, even if you release the dependent to them on form 8332.
