At one of our customers we have some trouble setting up a meetingroomaccount for teams. We made a resource mailbox and added a Exchange Online and a meetingroom license. When accounts from the same portal plans a meeting it shows up in the calendar of the teams app in the meetingroom account. But if external people try to plan a meeting it doesn't show up in the calendar. The teams setting for external access enabled.
I am not sure if the External User can send the meeting invite and that will show up on the calendar of the meeting room or not. As per my understanding only internal users can book a meeting room. But as a workaround you can try the below steps:-
External person will be doing a meeting with a in company user. In that case ask the in company user to forward the invite to the meeting room and this should work.