it is unlikely your strategy will work if you are serviced by any of the major servicers... you really can't 'pre-pay' interest. Your monthly payment is for the interest in the month just ended.
Payment due January 1 will be part of 2020 because you are paying December's interest
Payment due February 1 is unlikely to be 2020 interest because it's the interest for the month of January, 2021.
As @Opus 17 points out, some servicers will just hold that payment and not post it until February 1 comes about (so it won't be part of your 2020 interest payments) and others will take that payment that arrived very early and just pay down principle.
Suggest calling the servicer.
