While you have the right to use them, a 2012 license server won't automatically "know" how to handle a 2025 key. You often have to call the Microsoft Clearinghouse to have them manually downgrade the key for your legacy environment. Where to Buy Today
For legitimate licensing, you should look for authorized Microsoft Licensing Partners or reputable resellers: Windows Server 2012 R2 - Microsoft Lifecycle buy terminal server licenses 2012
Navigating the Modern Challenges of Buying RDS CALs for Windows Server 2012 While you have the right to use them,