When the XR18 is connected to an external router in DHCP mode, it will acquire an IP address from the router.
When Xair Monitor Mixer attaches to that same router, it should try to find the mixer like Behringer's apps do (presumably by sending UDP requests to the entire subnet) so that users don't have to manually enter an IP address.
It looks like you're new here. If you want to get involved, click one of these buttons!