If you have ever installed additional RDP licenses on a pre Windows 2012 server, you likely found this a straight forward procedure.
In server 2012 however, you will notice that the RDP Licensing Manager is not available.
To manage RDP Licensing, you will need to install Remote Desktop Licensing in the Remote Desktop Services section of the server role section. You can do the install by selecting Manage -> Add Roles and Features wizard from the server manager menu.
From there select Roles and locate Remote Desktop Service section and expand. Here you will find the Remote Desktop Licensing. Install this portion.
After installing the Remote Desktop Licensing ole, you can add enterprise licenses, however there is another caveat.
Once you have licenses installed, you need to either setup a GPO, or use gpedit.msc and enable/set the Use the specified Remote Desktop License Servers and the Set the Remote Desktop licensing mode options under Computer Configuration -> Administrative Templates -> Windows Components -> Remote Desktop Services -> Remote Desktop Session Host -> Licensing section of the mmc.
Use the specified Remote Desktop License Servers
When setting this option, select Enable and then look on the left side, lower section and you will see the option License server to use. In this field, enter the name of the license server, which for this article is the same system on which you installed the enterprise licenses, likely, this host.
Set the Remote Desktop licensing mode
When setting this option, select Enable, and set the Specify the licensing mode for the RD session host server option to either Per Device or Per User depending on how the licenses were installed.
When I setup the licenses initially, I did not specify the options, after the 120 day grace period, I was no longer able to logon to the server due to licensing errors.
Once the above options were set, RDP licensing began working again as expected.