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. If this post solves your problem please help others find this answer by clicking "Accept as Solution.".
Streaming Stick 4K+ 3821 | Streaming Stick+ 3810 | TCL Roku TV 43S245/C107X