Signing into Discovery Plus

You never actually use your Roku credentials on a Roku device to sign into anything.  Once it's linked to your Roku account, it's linked.  You need to use your Discovery+ credentials.  Are you subscribed to Discovery+ and if so, how?  There's multiple ways and if you've subscribed through Roku, you most likely need to access it via "The Roku Channel" app.  If you can sign into the Discovery+ website, you would use those same credentials to sign into the Discovery+ app on a Roku device.