@pcho -- what devices are connecting (via Ethernet or WiFi) to the XB8?
Is it possible that one of the connected devices is a third-party router, with one of its LAN ports connected to one of the LAN ports on the XB8? Only the WAN port on such a router should be connected to the XB8.
When you get a Windows computer that has received a "192.168.0.xxx" IP-address, open a command-line prompt, and enter the "arp -a" command, to see output like:
Interface: 10.0.0.152
Internet Address Physical Address Type
10.0.0.1 1c-9d-72-75-f8-48 dynamic
The ".152" is my computer's private IP-address, and the "10.0.0.1" is the private IP-address of my XB7 router, with its associated MAC-address. For your configuration, instead of each "10.0.0", you should see "192.168.0" for the "problematic" interface.
Compare the MAC-address that you see with the MAC-address of every device connected to your XB8, to try to find a match.
The first 6 letters of the MAC-address indicate which manufacturer's network-adapter is active in the router. There is a web-site that can accept those letters, and name the manufacturer, if that helps you.
Example: https://dnschecker.org/mac-lookup.php?query=1c-9d-72
I hope this helps.