If you have a valid product key but are having trouble activating, Microsoft recommends several official methods: Window server 2008 R2 Activation - Microsoft Q&A
To verify if the server is now genuine, use: slmgr.vbs /dli . 3. Phone Activation for Offline Servers
How to Activate Windows Server 2008 R2 Standard: A Complete Guide
If you're looking for alternatives or more information on server management, I'd be happy to help with that.
Here are some general steps for activating Windows Server 2008 R2 Standard:
Enter your 25-character product key and follow the prompts to activate online. 2. Activating via Command Line (SLMGR)