If secondary hub and primary hub are talking over tunnel, UMP needs to access to tunnel-proxy-port (in the primary hub).
Here is example.
Primary hub - tunnel server - 192.168.100.10 Secondary hub - tunnel client - 192.168.0.5
1.open hub GUI of the primary hub - [hubs] tab.
2.check [source] column for secondary hub.
3.it should look like
In this scenario, UMP robot needs access 48050 port in 192.168.100.10.