Seeing an USB modem appear only when the Uno is connected is generally a good sign. At this point you are probably thinking it is time to return your Uno. It may very well be.
However...
... the freezing serial monitor and lack of Network Device Detect pop-up window is suspicious to me.
Even more so with this statement:
hugigu:
I've even cleaned out traces of some old USB internet dongles I used some months ago and bluetooth is off.
"Internet USB dongles". Are those like 3G data card devices? These typically emulate a USBmodem. They are notorious for installing software / "drivers" that talk any device that looks like a usb-based serial port or usb-based modem.
Double check there are no traces of THAT software still around. (e.g. check in Sys Prefs -> Users -> Login Items).