If you didn't receive the check, there will be a section on your 2020 1040 tax form for "Recovery Rebate Credit" which seems to be the catch-all for anyone who missed the deadlines or didn't get their full amount for whatever reason.

I'm an American overseas who pays his taxes, has intent to return, yada yada and never received my check, and never got the online portal to recognize me, so I'm hoping this will work.

Source: https://www.irs.gov/newsroom/economic-impact-payment-informa...