Have you plugged into the router using Ethernet then done all the
updates ? One of these will be the Broadcom drivers and this should
sort it. If not, then the only option to get over the problem that I
know of is to change the wireless card to one that works. There are
some laptops with Broadcom wireless cards that just don't get on with
Ubuntu unfortunately but we are seen less problems as Ubuntu develops.
Broadcom devices used to be a nightmare to get working !!