People off-campus can't join

I've started using my university faculty Zoom Pro account for monthly meetings. (The account is linked to an email run by an administrator, not by me. When I need a code from Zoom, he sends it to me.)

People who came in-person to the meeting in a seminar room could join the Zoom meeting, but no-one off campus could join. I have no idea why. Outsiders got this message from Zoom: 'Host has joined, we've let them know you're here.' But I don't recall getting any pop-up messages to admit the several people who were waiting.

The problem makes our meeting look unprofessional and it is driving me potty. Any advice, please?

P.S. Don't reply by private message as that will go to the admin, who doesn't read English. (I work in Japan.)