By "signed up ... online" you mean directly with Paramount+ on their website, right?
It's been a while since I registered with Paramount+ so I don't really remember but ...
Try going into the Paramount+ app on your Roku and clicking on a program to watch. I *think* it will then give you the option to either subscript or sign in. Sign in with the credentials you originally set up with Paramount+. (Some channels do it this way, but I can't recall if Paramount+ is one of them.)
Roku Community Streaming Expert
I am not a Roku employee, only a user like you. Please, no support questions via private message -- post them publicly to the Community where others may benefit as well.
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Ultra 4800 | Streaming Stick 4K+ 3821 | TCL Roku TV 43S245/C107X
