If the SS# was issued AFTER you filed then you cannot amend a prior filed return for the CTC or EIC so that will be a waste of time. 

The Protecting Americans from Tax Hikes or PATH Act made several changes to the tax law, including preventing retroactive claims of the EITC and CTC by amending a return or filing an original return for any earlier year in which the individual or anyone listed on the return did not have a SSN valid for employment. You can't claim EITC or CTC unless the Social Security number for you, your spouse (if married filing a joint return) or a qualifying child is issued before the due date of the return including any valid extensions.

http://www.thetaxadviser.com/newsletters/2016/jan/congress-makes-changes-to-popular-tax-credits.html

As for getting the CTC in advance ....

The IRS was supposed to update their portal Manage Payments so a child born in 2021 could be eligible for the monthly Advanced Child Tax Credit payments in 2021.  This has not happened and there is a good chance it will not happen this year.

Go to this IRS website for the portal - https://www.irs.gov/credits-deductions/advance-child-tax-credit-payments-in-2021

Go to this IRS website for information concerning the portal updates - https://www.irs.gov/credits-deductions/2021-child-tax-credit-and-advance-child-tax-credit-payments-t...

You will be able to add your child as a dependent on your 2021 tax return and if eligible receive the Child Tax Credit on your 2021 tax return.

I am unsure exactly when the SS# was issued, but it was likely after the deadline. We delayed getting it for a long time as we live abroad, and it required us to travel to the nearest embassy. Didn't want to do that until vaccinated. 

Struggling to think of a good reason why they'd turn away people with an otherwise valid claim to that credit, particularly people just starting out with a new baby.  But I guess I can only blame myself for not knowing the rules. 

Thank you for the detailed response. Much appreciated.