If you’re getting the following error message upon connecting to your Windows Server 2019-based Remote Desktop Session Host that is not joined to a domain:
There is a problem with your Remote Desktop license, and your session will be disconnected in 60 minutes. Contact your system administrator to fix this problem.
…I’ve bad news for you: in Windows Server 2019 environment, User CALs require AD. If you insist on using RDS without AD, apparently you’ve to get Device CALs or downgrade to Windows Server 2016 or earlier (also: deleting the L$RTMTIMEBOMB registry key probably won’t help).
As Microsoft states, “CALs cannot be tracked within a workgroup” (yeah, right!). See also: Problem with Windows Server 2019 RDS License (Microsoft TechNet).