What about the firewall on the server? Is there a rule to allow all traffic to/from the UrBackup server process? I don’t have UrBackup Server running on windows at the moment to confirm what that looks like.
If so, this comes down to basic software/network troubleshooting. The server sends UDP packets to broadcast:35622. If you declare a Client discovery hint, it also sends those packets to the target’s IP directly. Use whatever means you have to ensure those packets are making it to the client computers.
If you are sure it is an issue with the UrBackup software and not firewall/network, attach your client log files per the instructions in this post.