As you found out, a license can only belong in a single team, and one team can only be linked to one domain.
I can’t think of a “best” answer, but as a compromise, you could link the team to the students domain (student.xxx.be) and then manually invite staff to the team as well, since presumably the staff would remain fairly consistent while students will frequently change. This arrangement would allow you to share a single lab license amongst your students and staff. It will mean though that your staff would have to be manually invited and have an account created if they do not already have one.