I had the same problem. My mistake was that I didn’t connect my remote stands to the service car stops. I’ve hired dozens of service agents and all they were doing was sitting in the staff rooms while the stands’ statuses were showing “boarding not started”.
I’ve tried to connect the service cars when setting up the whole system, but didn’t realize that you don’t connect the service car stops to the stands, instead you have to connect the stands to the stops. After doing that I had to dismiss a flew flights in a row to get rid of the backlog and everything was fine. Well, and unfortunately fire a few dozens of service agents.
Hope that helps someone coming here looking for a solution like I did. Stay healthy and fly safe.