How do I license remote desktop servers without Active Directory?

My company provides some of our clients with remote desktop servers with which to run our physical software. Each server is its own virtual machine hosted on AWS, and each user on the machine is a local user, not an active directory unit. None of our servers are on Active Directory.

This is creating a problem when attempting to provide licensing for each of these users to connect through RDP. Historically I guess we had some workaround to allow for these non-AD users to be licensed to use RDP, but we're not able to achieve this any longer. The official Microsoft documentation seems to indicate, at least for per-user CALs, that active directory is required to license RDP sessions.

We would really like to avoid switching to Active Directory for a multitude of reasons. Not least of which; it would require us to re-create and switch over every single one of our hundreds of local users. Is there any way we can effectively license each of these servers to support RDP sessions into local user sessions?

Extra info:

• All of our servers are running Windows Server 2016

One thought I had was: can we join a domain with each server to get the RDP sessions licensed, but still log into the RDP session with local users? That way we wouldn't have to re-create all the users.