"It's not personal" - Roku does not remember any SSIDs, except the one in use.
Hiding an SSID is such a terrible idea, I'd be inclined to put up a big banner that says "Please restore your router to a non-stupid state before continuing".
Luckily I'm not in charge of such decisions.
What you describe as private network is a "hidden SSID".
It's the WiFi owner's choice whether to use such feature. Ask them to fix it or live with it - it's not real security but privacy-through-obscurity.