OK, Kelsos, now it seems to be working fine!!
I decided to try with the Remote app on my tablet (I was using the app on my mobile phone) to see if it made any difference, but I was pessimistic. I ran MB on the portable installation in my USB drive, and suddently there was this standard dialogue box to allow MB or the remote (or both) to connect to private and public networks.
I think I had seen this message before, but certainly not today, and I was testing a lot. I don't know if it was a coincidence that it popped when I used the app from my tablet.
Any idea what to do if I need to do a new installation again some other time? Can this dialogue box be forced to be displayed?
I'm sorry if I've made you work on this for nothing! Thanks anyway!