You should file a Form 8606 for 2017 and 2018 (assuming your applied your 4/11/2019 contribution to 2018). You don't have to file an amended tax return, you can just file the Form 8606 by itself.
Be sure to sign the form at the bottom of page two and use the correct year's form.
Click here for 2017 forms.
Click here for 2018 forms.
Technically you can be fined $50 for failing to file the form on time, but the penalty can be waived if you have reasonable cause.
Click here to learn more about reasonable cause.
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