The default PIN is 0000 - have you tried that?
If not, I think that you need to have it reset remotely. You would be best to try the Messaging service from My Optus App. Be aware that they are running a couple of days behind due to COVID19.
If you didn't want to wait, then you could do a hard reset to return to factory default.
But make sure that you have your activation code handy before doing so.