I don’t know which interface would use UrBackup client if PC has 2 interfaces at same network (as wifi and ethernet). I didn’t see any information about it at the documentation. May be @uroni write answer about it.
One of the possible problems:
UrBackup server discovers clients only in the same sub-network. Server sends UDP broadcasts for discovery by 35623 udp. Broadcast messages from UrBackup server goes only in same sub-network. Client listen it and answers.
Documentation: 5 Client discovery in local area networks
Probably your wireless interface gets this broadcast first and answers first.
Disable your wireless device at device manager. After it your OS will see only one ethernet interface.
Reboot PC and check it again.
If UrBackup server will see this client you will know that problem not at the your ethernet interface.
If it help enable your wireless interface, enable FW.
Block UDP ports 35622, 35623 only for wireless interface at windows FW.
After it UrBackup server can’t connect to your wireless interfaces.