Recently upgraded a 2008 R2 Server to Enterprise from Standard using

 Dism /online /Set-Edition:ServerEnterprise /ProductKey:xxxxx-xxxxx-xxxxx-xxxxx-xxxxx

Wouldn’t work with the customers Volume License Enterprise Key. I used the KMS key which did work and activated using the Volume license key again , however on restart the server could not be remote desktoped in.

I had to run a 

SLMGR /REARM

Restart the server and reactivate it using the proper Volume license key. After this , restarting the Remote Desktop Service enabled RDP to work again

You should check in the Computer settings , that the memory is not set to 4GB Usable out of the new memory allowance. If this is the case you will need to do the following

1. Open the command prompt and change working directory to System32 directory
2. Type in slmgr.vbs -ckms (this clears and KMS entry you may have)
3. Type slmgr.vbs -upk (this removes any product key installed)
4. Type slmgr.vbs -ipk xxxxx-xxxxx-xxxxx-xxxxx-xxxxx (where the x’s is the new product key you want to use )
5. Type slmgr.vbs -ato (this activates the server)
6. Restart Server