What if I can’t remember the password to an old device?

My old iPhone keeps being disabled because I can’t remember the 4 digit passcode to unlock it. I kept it as a backup phone and am needing it but can’t get it unlocked. I typically use the same 4 digit passcode on all of my devices but for whatever reason, my usual 4 digit one isn’t working on it. Not sure what else it could be. It’s clearly my device as it’s registered to me. How do I go about unlocking it?