You can go with CAL.
Purchase User CALs for each of the 20 external developers. This allows them to access the server from any device they use. Each user connecting to the server needs a User CAL.
This article describes how to move Terminal Services Client Access Licenses (CALs) from one Terminal Services license server to another in Windows. https://learn.microsoft.com/en-us/troubleshoot/windows-server/remote/move-terminal-services-client-access-licenses
Hope this resolves your Query !!
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