Until that portal is open any DD the IRS sends that is rejected by the bank for any reason will be converted to a paper check mailed to the address on the last filed processed return.
Sometime in August the IRS will open a portal so you can change banking information for the monthly CTC payments. That portal does not work yet.
https://www.irs.gov/credits-deductions/advance-child-tax-credit-payments-in-2021
