It's possible that your WiFi dongle is bad. However, before coming to that conclusion, let's try a couple of things:
Run the Reset WiFi command (Maintenance menu). The Brennan website recommends plugging the WiFi dongle into USB A. Power down, switch the dongle to USB A, wait another 20 seconds, and power up again. Try the Setup WiFi command. Does the display show available networks? If it shows your network - the SSID of your router - go ahead and complete the WiFi setup process. Either way, please report back here what you find.
The USB B slot is non-functional for any device. It was only there for early units that did not have an internal hard disk drive. The internal HDD now uses USB B, and the latest models do not have that port shown on the back panel.