First you can still file a joint return since you are still married and you can only use HOH if you did not live together anytime during the last 6 months of the year.
Next this advance CTC payments are so new the IRS FAQs on this subject have not yet been updated ... so don't rush to file since any return with the CTC on it will not process before the end of February and ones with advance payments could take even longer. https://www.irs.gov/individuals/understanding-your-letter-6417
https://www.irs.gov/credits-deductions/2021-child-tax-credit-and-advance-child-tax-credit-payments-f...
