The player FragiPL found this awesome solution and can confirm that works like a charm.
Well, based on the FragiPL solution, step by step:
Put the console offline
Reset console (restart)
Change the date to 21 October 2018
Start the game offline* and the Roman mile PR Stunt should appear. Just beat it. (Its a speed trap located at top left on the map)
Put the console online, start the game and achievement should pop up.
I used the gamepass copy.
Hope that helps.
*Update: apparently some people need to set home console in order to run the digital copy offline.