When you flashed the firmware, it overwrote all settings you'd made before. That means that your network name, any passwords, channels, etc., and whether your network was set for infrastructure or ad hoc. What it looks like is that all you need to do is restore those settings to match your computer's AirPort settings. Have you tried this yet? Open the Admin Utility on your computer and see what each setting is, then make the settings on configuration pages for the router match them.