In general, enabling UPnP on a router is only done to allow “Outside In” connectivity mapping.
If your WiFi PC is on the same internal network, enabling or disabling UPnP on the router should have no bearing on the PC’s ability to discover the MBL.
But there are plenty of routers out there that prevent WiFi clients from accessing things on the wired network.
Make sure you’re not inadvertently using “Guest” networking functions on your WiFi router.
Also, if your MBL is using a Dynamic IP, look in your router to determine what IP address the router assigned to the MBL, and then access that IP address directly using the URL
http://192.168.1.122 (or whatever.)