Many users seek version 5.20 specifically because it is one of the last versions for which "cracked" or pre-activated ISO files are readily available. These modified files allow users to run a full-featured router on a standard PC without purchasing a formal license from Security and Compatibility Risks Malware Risks:

before moving to the latest stable version to ensure configuration stability. MikroTik community forum

A MikroTik 5.20 license key is used to activate RouterOS version 5.20

and click "Paste Key" or "Import Key". A reboot is required for changes to take effect. Why Version 5.20 is Still Used

A 24-hour trial; the system will stop working after this period if not registered.