Here's what the instructions for the relevant line on form 1040 say:
Enter the total amount paid in 2018 for health insurance coverage established under your business (or the S corporation in which you were a more-than-2% shareholder) for 2018 [...]
Oddly, this says both "paid in 2018" and "coverage for 2018". This would seem to indicate that payments made in December 2018 for coverage in January 2019 would actually not be deductible in either year! (They wouldn't be deductible in 2019 because they were paid in 2018, but they wouldn't be deductible in 2018 because they were paying for coverage for 2019.)
That is probably not right, but I think it'd take someone with deeper knowledge than me to fully explain the right answer.
The common-sense approach would be to claim 2018 deductions based on what is shown for the 1095-A you received for 2018. At the very least, if someone comes after you, you have a form you can point to that purports to tally up your premiums for 2018 and you can plausibly argue that you acted in good faith based on that document. That means you would only take the deduction for the payment you made in November 2018 (coverage for December 2018). You will have to hope that when you receive your 1095-A next year, it correctly shows what you paid for that January 2019 coverage. If it doesn't, perhaps at that time you can claim the deduction for your January 2020 coverage.
I'm assuming here that the premium amount for this one month is not large enough to really swing things hugely either way. Also, of course, I'm not a tax expert and this is only my common-sense layperson's reading of the instructions.