Sometimes things are not done properly and you have to correct the situation. So, to successfully convert a trial version into a licensed version for production, you can run the following DISM command line:

dism /Online /Set-Edition:ServerStandard /AcceptEula /ProductKey:XXXX-XXXX-XXXX-XXXX
Use the Set-Edition:ServerDatacenter for the Windows Server Datacenter edition with the appropriate client key.

The operation is not so trivial because the Microsoft-Windows-ServerStandardEvalEdition package is removed from the system. However, it can be performed online without impact. Note that the processus is stuck for 20/30 minutes at 10% but finally is done successfully and that it is necessary to reboot the system at the end. It seems that if you disconnect from Internet, the job is done more quickly.

Hope this help and to learn more on Windows Server 2016 Activation, click this link on Microsoft Docs,