Under normal circumstances cross-border workers fall under the social security system of their country of residence if they spend more than 25% of their total working time there.
These regulations were temporarily halted during the pandemic. But they will likely return next year. Cross-border workers would also be subject to paying taxes in their country of residence after a certain number of days worked at home per year: 29 for those in France, 19 in Germany and 34 in Belgium.