Just to make sure: you mean you have enabled the OEM unlock in the developer options?
That is not the actual unlock. It just allows you to do the unlock.
If you say you cannot proceed, does that mean the installer hangs in a screen like this?
As that is a typical problem when installing using a Windows PC I assume you’re on Windows. Is that correct?
In short: if you are on Windows and that’s the screen where you can’t proceed, here is the most likely solution: [HOWTO] Fairphone 3/3+ stuck in fastboot/bootloader mode with the Easy Installer on Windows - HOWTOs - /e/OS community