Got a new router from my provider today. All my other wireless devices (tablet, printer, etc.) connected up fine to the new router, but for some reason my Ubuntu laptop wouldn't.
When I plugged in an ethernet cable, everything was fine, DHCP was working fine.
Eventually a noticed that for some reason the DHCP lease for the Wifi was not clearing and I was getting an old IP address.
Eventually via some simple Google searches learned that you should clear old DHCP leases in /var/lib/dhcp.
But for some reason that wasn't working ... what else was I missing?
After some trial and error, the key was also clearing the leases stored in /var/lib/NetworkManager.
Hopefully this proves useful for someone out there in the Internet. I'm sure there are some pretty GUI based ways to do this, but I'm not a GUI person and was unaware of how to do it that way.