If If you don’t have the exact registration number or the website doesn’t recognize it, you can validate your PIN with; your last name (first option), first name or middle name, or instead use your date of birth and state of origin… type either instead of the registration number… check the note under the registration number input:
"Note: For those who may have forgotten their Registration Number, this can be retrieved upon validation of PIN and Serial Number with either surname, first name or middle name, Date of Birth and State of Origin".