If the military taxpayer is on active duty stationed outside the US and Puerto Rico they have an automatic 2 month extension to file a 2018 tax return on or before June 17, 2019.  If additional time is needed to file the 2018 tax return then the taxpayer must file a Form 4868, Application for Automatic Extension of Time To File U.S. Individual Income Tax Return on or before June 17, 2019 for an extension to October 15, 2019.  If the form 4868 is not filed then the due date of the 2018 tax return is June 17, 2019.

Since you did not file a Form 4868 and the child did not receive a SS number until after the due date of the return the IRS will deny any Earned Income Tax Credit, Child Tax Credit, Additional Child Tax Credit and Other Dependent Credit for the 2018 tax return based on the child being a dependent.

If your dependent child did not receive his/her SSN until after the due date of your return, option one is to file an amended return that includes her SSN.   See this IRS reference:

https://www.irs.gov/faqs/filing-requirements-status-dependents/dependents/dependents-9

Sadly the IRS is correct...you did not file a timely return or extension so the child tax credit is not allowed due to the new rules installed a couple of years ago. You could beg for a reconsideration but I doubt it will work.