Originally Posted by Dan203
To disable RF you hold TiVo and C for a few seconds. Or maybe TiVo and D. Can't remember which. But once it changes the light will flash red instead of yellow when you press a button. After that it should work for programming your Roamio.
My way of remembering is "it's the RED button to switch back to infrared mode." (though it would have been even more sensible if TiVo had used the A/Yellow button to revert to RF mode, to match the LED flash color, rather than D/Green)
As for the OP's issue, yeah, it would probably be best to put the Roamio remote into IR mode, for testing and any IR training/learning that might be needed. Once the remote is in IR mode, you should also head to the System Information screen, to determine the Roamio's configured "Remote Address."
You'll likely want to make sure the Roamio is set to the default "Remote Address" of 0, at least initially, to get through the Harmony programming issue you're having.