FMS v21 and FMP v21 Upload to host

Is there a specific port that FMP uses to upload a database to FMS?

My situation is:

Legacy MacMini w/FMS v19

New MacMini w/FMS v21

Both computers are in the client office location and static DHCP addresses assigned. User access the database via a VPN connection.

My user is trying to upload a test database to the New MacMini via FMP via Upload to Host. The result is an endless 'Waiting for response'

The user tested Upload to Host to my FMPHost server and the credentials dialog appear right away. Obviously my FMS is outside the client's network.

I suspect one or more of the FM ports are closed, however their client's IT says that all ports are open.

If I had a $1 for every time IT told me all ports are open...