If you were HSA-eligible every month of 2019 you would not be using the last-month rule to determine your HSA contribution limit for 2019. The last-month rule is only potentially used by those who are HSA-eligible on December 1 but not every month of the year.
Because you were HSA eligible for every month of 2019, you have until April 15, 2020 to make a contribution for 2019 up to the maximum permissible for the type of HSA coverage (self-only or family) you had during the year.
