I live in Brussels myself and usually go to an Exki-restaurant when in between client meeting. Exki is a chain of healthy 'pret a manger-style' restaurants. Their Wifi is fast, reliable and unlimited. Peak hours aside, the atmosphere tends to be laidback as well. My favourite one is at Porte de Namur but they have plenty of other locations in both uptown and downtown Brussels. For a complete list, have a look at http://exki.be/default/en-be/restaurants/belgium/bruxelles.aspx
This tip has the address "Chée d'Ixelles 12, 1050 Brussels"