You can file the missing 2018 8606 by itself.
The 2018 contribution should be in line 1 prior year (if any) on line 2 and total on line 3 & 14.
https://www.irs.gov/pub/irs-prior/f8606--2018.pdf
https://www.irs.gov/pub/irs-prior/i8606--2018.pdf
https://www.irs.gov/filing/where-to-file-addresses-for-taxpayers-and-tax-professionals-filing-form-1...
**Disclaimer: This post is for discussion purposes only and is NOT tax advice. The author takes no responsibility for the accuracy of any information in this post.**
