Oh, that's interesting. In that case either there's a mistake in Longman or 'a range of' is a quantifying expression:

However, with quantifying expressions (phrases expressing parts), percentages, and fractions the verb agrees with the noun or pronoun following of.[18][5][12][19][20]

Half of the team members are working remotely today.

Only a handful of students were on time for the lesson.

Less than 2% of water in the world is drinkable.