Hello,
If the server doesn't have internet, you may activate the RDS cal via https://activate.microsoft.com/Default.aspx?locale=en-US on another computer which has internet. The URL for the Web installation method is displayed in the Install Licenses Wizard. Note the address for the Terminal Server Licensing Web site, and then connect to the Web site from a computer that has Internet connectivity.
Detailed steps please refer to:
Install Terminal Services Client Access Licenses by Using a Web Browser | Microsoft Learn
If you do not have Internet connectivity, you can also contact the Microsoft Clearinghouse by telephone. For more information, see Locate the Microsoft Clearinghouse Telephone Number for Your Country or Region.
Install Terminal Services Client Access Licenses by Using the Telephone | Microsoft Learn
If the Answer is helpful, please click "Accept Answer" and upvote it.