get 2019 Form 1040-X from IRS website and mail it in with your 2018 Form 8606, which you can also get in fillable PDF.
https://www.irs.gov/pub/irs-prior/f1040x--2019.pdf
Note: since you are not changing any dollar amounts on your amended tax return, you can leave all the lines 1-23 EMPTY.
Part III explanation: "didn't include Form 8606 with original return". OR whatever other reason.
mail to the address in the latest Instructions for Form 1040-X
@bondtradercu