How do I claim the AdvCTC payments as the custodial parent after a divorce? Our decree stated I get 100% of CTC but both my ex and I pymt ltrs from IRS for 50% each.

Which one of you is the custodial parent?   The IRS cares who the child lives with the most---you may even need to count the nights--whoever had the child for 183 nights or more is the custodial parent.   That parent is the one who can file as Head of Household, get the childcare credit and earned income credit IF there is a signed form 8332.   The non-custodial parent gets the child tax credit, but cannot use the child to file as HOH, get childcare credit or earned income credit.

As for who puts what on their tax return for the advance CTC payments that went out from July to December---you got bum advice.   The person who RECEIVED those payments enters those payments on their 2021 tax return.   If you are not the one who got those payments they do not go on your tax return.   If the person who received the advance payments is not claiming the child on a 2021 tax return, they may or may not have to pay it back, depending on their 2021 income.

The person who is claiming the child for the child tax credit on a 2021 tax return gets the CTC on line 28 of their form 1040.   

The IRS is not going to sort out confusion between parents who received the checks and turned the money over to the other parent, etc.   Each of you enters the amount you personally received.   Enter your letter 6419 correctly if you have that letter.   Do not enter information from someone else's letter.