you may not mix RDS 2016 and RDS 2012 R2

you may not mix RDS 2016 and RDS 2012 R2
https://technet.microsoft.com/en-us/windows-server-docs/compute/remote-desktop-services/rds-in-azure

https://technet.microsoft.com/en-us/windows-server-docs/compute/remote-desktop-services/rds-supported-config

• Use Windows Server 2016 for your Remote Desktop infrastructure - the Web Access, Gateway, Connection Broker, and license server. Windows Server 2016 is backward compatible for these components - so a 2012 R2 RD Session Host can connect to a 2016 RD Connection Broker, but the reverse is not true.
• For RD Session Hosts - all Session Hosts in a collection need to be at the same level, but you can have multiple collections. You can have a collection with Windows Server 2012 R2 Session Hosts and one with Windows Server 2016 Session Hosts.
• If you upgrade your RD Session Host to Windows Server 2016, also upgrade the license server. Remember that a 2016 license server can process CALs from all previous versions of Windows Server, down to Windows Server 2003.