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 Streaming Stick +, 3810X, Ser YH0059427035, wifi - Samsung UN55ES6100
Roku Streaming Stick, 3600X, Ser 5S56D8240827, wifi - JVC EM37T