The achievement requires you to finish an online race. It needs to be an actual race session and not qualifying or free practice.
What's not included in the description is that you need to take the checkered flag in order to unlock the achievement. Therefore you need to cross the finish line within the time limit, after the winner of the race crosses the line.
If you're struggling, you can easily boost this with a friend by creating an online event of your own, with only two participants.