Remote learning has reduced the mathematical performance of students. Mathematical reasoning is the critical skill that enables students to make use of all other mathematical skills. The aim of the present study was (1) to develop the mathematical reasoning skills of underachieving students and (2) to explore the application options, benefits and limitations of an online game-based intervention programme among third- and fourth-grade pupils (aged 9-11, N = 810). The content of the programme was designed to strengthen their basic maths skills in line with the curriculum. Beyond assigning the tasks, the digital intervention programme also performed the motivational, differentiation- and feedback-related tasks of the teacher. The reliability indices for pre-, post and follow-up test results proved to be high (Cronbach's alpha = 0.90, 0.91 and 0.92, respectively). The effect size of the programme proved to be significant in both grades (d = 0.22 and 0.38, respectively). The results confirm the potential of the intervention programme to close, or at least significantly reduce the COVID learning gap in basic maths skills, without the need for additional teacher work--which is an important aspect of successful implementation--in areas which are the most challenging for 9-to-11-year-old pupils in the domain of mathematics.