After I sent the email to the reviewer, I wanted to send a reminder again, but I didn’t find the reminder button. I remember it existed before
It might not be there after its used initially - I’m not sure. But you do have the capability to email the reviewer within the submission (by clicking the arrow next to their name):
Please note: 3.1 is an older version and is no longer supported (we won’t always be able to address questions related to 3.1 here on the forum), so I’d recommend that you’d upgrade when you get the chance.