Welcome to Android Central! If you did a Remote Lock using Android Device Manager, that may not help if the phone already has a PIN or password set. A lot of times, you'll get a message saying something like, "Your phone already has a password set." According to the Google support page, Remote Lock "locks your device with a password if you don't already have one set."
You mentioned never using a PIN before, and then suddenly it was asking for one. Were you using a fingerprint unlock beforehand (if your phone has that feature)?