Normally you just attach an antenna to the RokuTV and perform a Channel Scan. (Settings...TV Inputs..LiveTV...Scan for Antenna Channels...Start Finding Channels).
To watch the over-the-air channels (OTA), you then select the Antenna Input from the Home Screen. (If the Antenna Input is missing, then Settings...TV Inputs.. [Look for Inputs not Used and Select LiveTV].
Not sure if I am missing something in your original question. If you think you are not getting enough channels then perform the Channel Scan again. The major networks are going to be part of your local OTA channels. (ie. ABC, NBC, Fox, CBS, etc)
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