I was getting the child tax credit payments in 2021 and ex-spouse paid off back child support

Yes, you will have to pay it back unless your income is low enough for you to qualify for repayment protection. See the questions and answers at the following link for details.

Topic H: Reconciling Your Advance Child Tax Credit Payments on Your 2021 Tax Return

You would have to repay any advance child tax credit payments you received if you did not claim the credit on your tax return. However, if the children lived with you, often you would be able to claim the child tax credit regardless of the amount of child support money you received.

I suggest you go through the child tax credit section in TurboTax and answer the questions to see if you can qualify.

Be aware that if you are the custodial parent he can’t claim the child unless you give him a signed form 8332. 

Repayment of the credit depends on your income. 

If your modified AGI for 2021 doesn't exceed $40,000 (single filers), $50,000 (head-of-household filers), or $60,000 (joint  filers), and your principal residence was in the U.S. for more than half of 2021, you won't have to repay any overpayment amount.

. If your modified AGI for the 2021 tax year is at least $80,000 (single filers), $100,000 (head-of-household filers), or $120,000 (joint filers), you have to pay back your entire overpayment.

Repayment for those in the middle is based on a formula. TurboTax will calculate any repayment for you.

OK, if you are going to allow the other parent to claim the child, you need to give them a form 8332.  You will still qualify for head of household status, EIC and the dependent care credit (if you qualify), because those benefits can't be waived, transferred or shared.  No matter what your custody agreement or divorce order says, your ex can't claim the child unless you give them that form, unless your ex lies and says the child lived with them more than half the year.  

If you received advance payments for a dependent and then don't claim the dependent on your tax return, you may have to repay the credits.  If you are lower income, you may qualify for repayment protection, see the questions and answers at this link.

https://www.irs.gov/credits-deductions/2021-child-tax-credit-and-advance-child-tax-credit-payments-topic-h-reconciling-your-advance-child-tax-credit-payments-on-your-2021-tax-return

However, if you also received a $1400 stimulus for the child, you don't have to repay that, even if you don't claim the child on your tax return.