Urbackup client in another subnet



I’m trying to connect a urbackup client from a different subnet to the Urbackup server.

These are my specifications.

-we have 2 subnets in our network
-the connection to the other subnet goes via vpn
-the urbackupclient can not be added automatically
-when the urbackup client was added manually (with hostname or with ip-address), the client is in an seperated list shown as online
-there is no dns between the two subnets
-the firewall allows everything between the two subnets
-the logs on the urbackup server and urbackup client did not show anything, what could be helpful

But the urbackup client is still not recognized. The status of Urbackup client is idle. Do you have any idea, what else I could try?

Thanks for your support,