I believe you can bring the RDS Client Access Licenses (CALs) to get around 2 TS connections. Unfortunately I cannot find the pointer to the right documentation on how to do this. You should be able to use the same process that you would use on a standard
Windows 2016 / 2012 Server on the DSVM Server which is also based on Windows 2016 or 2012 Server in Windows.
For Devtest labs you need to go into settings and enable marketplace images as indicated in the DevTest labs doc: https://azure.microsoft.com/en-us/updates/azure-devtest-labs-use-azure-marketplace-images-to-create-vms-in-the-lab/
