I suggest you go to the source and contact FMI support.
In the US:
+1 408-727-8227 (Outside of North America)
Mon. - Fri. 7:00 AM - 5:00 PM Pacific time (holidays excluded)
They can validate the License Key and your contract to make sure you have what you paid for.
I've spoken with Filemaker, they say it should work but to test the serial number they suggest installing a demo version of FM server and testing connecting multiple users on an in house network. Obviously quite a big job and in some ways not hugely helpful as it is not how we are using the database.
Just wondered if anyone knew of something behind the scenes, say a setting in the database itself that needs to be active. Or other troubleshooting. Or if this is just not possible at all.
The concurrent connections apply to server connections. Not multiple installations of FMP. It's legal to have a copy on your desktop machine and laptop but they cannot be used at the same time on the same network. That is if I understand your question . . .