TV's that don't have HDMI are just increasingly useless in 2021 and onwards. Really time for a new TV that has HDMI. Not sonething Roku can fix.
A few older Roku models do have RCA but not really enough processor or memory for latest OS and channels, and will be desupported soon, so I would not bother trying to buy one secondhand on eBay or similar, unless your watching needs are really basic.
HDMI standards put limitations on converting down to older video standards, for content protection reasons. Will work sometimes but often not. Game over, thanks for playing if that kind of connection is not working to attach Roku. Buy a new TV that supports HDMI.
Nothing to do with TV being Plasma, just old and not having modern ports.
Not a Roku employee