A lot of what you're saying doesn't make sense. First, the 3920 wasn't dual band, so I don't know how it could have been connected to 5GHz. I've seen other people claim that their 2.4GHz-only Roku was connected to 5GHz, so I don't know what to think about that one. Second, if you unplug your router and your Roku still shows your 2.4GHz SSID then there's really something strange going on. Once you unplug the router, reboot/restart the Roku. If it still shows your 2.4GHz SSID then something is broadcasting that SSID.
Go into Settings->System->Advanced system settings and select "Network connection reset".
Finally, sorry, there's no way to connect it to a computer to update it.
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